Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/dyn/compute_v1.instances.html b/docs/dyn/compute_v1.instances.html
index f3db40d..340a16f 100644
--- a/docs/dyn/compute_v1.instances.html
+++ b/docs/dyn/compute_v1.instances.html
@@ -99,7 +99,7 @@
   <code><a href="#get">get(project, zone, instance)</a></code></p>
 <p class="firstline">Returns the specified Instance resource. Get a list of available instances by making a list() request.</p>
 <p class="toc_element">
-  <code><a href="#getSerialPortOutput">getSerialPortOutput(project, zone, instance, port=None)</a></code></p>
+  <code><a href="#getSerialPortOutput">getSerialPortOutput(project, zone, instance, start=None, port=None)</a></code></p>
 <p class="firstline">Returns the specified instance's serial port output.</p>
 <p class="toc_element">
   <code><a href="#insert">insert(project, zone, body)</a></code></p>
@@ -204,7 +204,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -285,7 +285,7 @@
                   },
                   "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
                   "diskName": "A String", # Specifies the disk name. If not specified, the default is to use the name of the instance.
-                  "sourceImage": "A String", # The source image used to create this disk. If the source image is deleted, this field will not be set.
+                  "sourceImage": "A String", # The source image to create this disk. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
                       #
                       # To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
                       #
@@ -302,6 +302,8 @@
                       # You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name:
                       #
                       # global/images/family/my-private-family
+                      #
+                      # If the source image is deleted later, this field will not be set.
                   "diskType": "A String", # Specifies the disk type to use to create the instance. If not specified, the default is pd-standard, specified using the full URL. For example:
                       #
                       # https://www.googleapis.com/compute/v1/projects/project/zones/zone/diskTypes/pd-standard
@@ -320,7 +322,11 @@
                 "mode": "A String", # The mode in which to attach this disk, either READ_WRITE or READ_ONLY. If not specified, the default is to attach the disk in READ_WRITE mode.
                 "interface": "A String", # Specifies the disk interface to use for attaching this disk, which is either SCSI or NVME. The default is SCSI. Persistent disks must always use SCSI and the request will fail if you attempt to attach a persistent disk in any other format than SCSI. Local SSDs can use either NVME or SCSI. For performance characteristics of SCSI over NVMe, see Local SSD performance.
                 "type": "A String", # Specifies the type of the disk, either SCRATCH or PERSISTENT. If not specified, the default is PERSISTENT.
-                "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. This field is only applicable for persistent disks. Note that for InstanceTemplate, it is just disk name, not URL for the disk.
+                "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
+                    #
+                    # If desired, you can also attach existing non-root persistent disks using this property. This field is only applicable for persistent disks.
+                    #
+                    # Note that for InstanceTemplate, specify the disk name, not the URL for the disk.
               },
             ],
             "name": "A String", # The name of the resource, provided by the client when initially creating the resource. The resource name must be 1-63 characters long, and comply with RFC1035. Specifically, the name must be 1-63 characters long and match the regular expression [a-z]([-a-z0-9]*[a-z0-9])? which means the first character must be a lowercase letter, and all following characters must be a dash, lowercase letter, or digit, except the last character, which cannot be a dash.
@@ -463,7 +469,7 @@
     },
     "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
     "diskName": "A String", # Specifies the disk name. If not specified, the default is to use the name of the instance.
-    "sourceImage": "A String", # The source image used to create this disk. If the source image is deleted, this field will not be set.
+    "sourceImage": "A String", # The source image to create this disk. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
         #
         # To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
         #
@@ -480,6 +486,8 @@
         # You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name:
         #
         # global/images/family/my-private-family
+        #
+        # If the source image is deleted later, this field will not be set.
     "diskType": "A String", # Specifies the disk type to use to create the instance. If not specified, the default is pd-standard, specified using the full URL. For example:
         #
         # https://www.googleapis.com/compute/v1/projects/project/zones/zone/diskTypes/pd-standard
@@ -498,7 +506,11 @@
   "mode": "A String", # The mode in which to attach this disk, either READ_WRITE or READ_ONLY. If not specified, the default is to attach the disk in READ_WRITE mode.
   "interface": "A String", # Specifies the disk interface to use for attaching this disk, which is either SCSI or NVME. The default is SCSI. Persistent disks must always use SCSI and the request will fail if you attempt to attach a persistent disk in any other format than SCSI. Local SSDs can use either NVME or SCSI. For performance characteristics of SCSI over NVMe, see Local SSD performance.
   "type": "A String", # Specifies the type of the disk, either SCRATCH or PERSISTENT. If not specified, the default is PERSISTENT.
-  "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. This field is only applicable for persistent disks. Note that for InstanceTemplate, it is just disk name, not URL for the disk.
+  "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
+      # 
+      # If desired, you can also attach existing non-root persistent disks using this property. This field is only applicable for persistent disks.
+      # 
+      # Note that for InstanceTemplate, specify the disk name, not the URL for the disk.
 }
 
 
@@ -548,7 +560,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -607,7 +619,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -668,7 +680,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -728,7 +740,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -790,7 +802,7 @@
           },
           "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
           "diskName": "A String", # Specifies the disk name. If not specified, the default is to use the name of the instance.
-          "sourceImage": "A String", # The source image used to create this disk. If the source image is deleted, this field will not be set.
+          "sourceImage": "A String", # The source image to create this disk. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
               #
               # To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
               #
@@ -807,6 +819,8 @@
               # You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name:
               #
               # global/images/family/my-private-family
+              #
+              # If the source image is deleted later, this field will not be set.
           "diskType": "A String", # Specifies the disk type to use to create the instance. If not specified, the default is pd-standard, specified using the full URL. For example:
               #
               # https://www.googleapis.com/compute/v1/projects/project/zones/zone/diskTypes/pd-standard
@@ -825,7 +839,11 @@
         "mode": "A String", # The mode in which to attach this disk, either READ_WRITE or READ_ONLY. If not specified, the default is to attach the disk in READ_WRITE mode.
         "interface": "A String", # Specifies the disk interface to use for attaching this disk, which is either SCSI or NVME. The default is SCSI. Persistent disks must always use SCSI and the request will fail if you attempt to attach a persistent disk in any other format than SCSI. Local SSDs can use either NVME or SCSI. For performance characteristics of SCSI over NVMe, see Local SSD performance.
         "type": "A String", # Specifies the type of the disk, either SCRATCH or PERSISTENT. If not specified, the default is PERSISTENT.
-        "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. This field is only applicable for persistent disks. Note that for InstanceTemplate, it is just disk name, not URL for the disk.
+        "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
+            #
+            # If desired, you can also attach existing non-root persistent disks using this property. This field is only applicable for persistent disks.
+            #
+            # Note that for InstanceTemplate, specify the disk name, not the URL for the disk.
       },
     ],
     "name": "A String", # The name of the resource, provided by the client when initially creating the resource. The resource name must be 1-63 characters long, and comply with RFC1035. Specifically, the name must be 1-63 characters long and match the regular expression [a-z]([-a-z0-9]*[a-z0-9])? which means the first character must be a lowercase letter, and all following characters must be a dash, lowercase letter, or digit, except the last character, which cannot be a dash.
@@ -896,22 +914,25 @@
 </div>
 
 <div class="method">
-    <code class="details" id="getSerialPortOutput">getSerialPortOutput(project, zone, instance, port=None)</code>
+    <code class="details" id="getSerialPortOutput">getSerialPortOutput(project, zone, instance, start=None, port=None)</code>
   <pre>Returns the specified instance's serial port output.
 
 Args:
   project: string, Project ID for this request. (required)
   zone: string, The name of the zone for this request. (required)
   instance: string, Name of the instance scoping this request. (required)
+  start: string, For the initial request, leave this field unspecified. For subsequent calls, this field should be set to the next value that was returned in the previous call.
   port: integer, Specifies which COM or serial port to retrieve data from.
 
 Returns:
   An object of the form:
 
     { # An instance's serial console output.
+    "start": "A String", # [Output Only] The starting byte position of the output that was returned. This should match the start parameter sent with the request. If the serial console output exceeds the size of the buffer, older output will be overwritten by newer content and the start values will be mismatched.
     "kind": "compute#serialPortOutput", # [Output Only] Type of the resource. Always compute#serialPortOutput for serial port output.
     "selfLink": "A String", # [Output Only] Server-defined URL for this resource.
     "contents": "A String", # [Output Only] The contents of the console output.
+    "next": "A String", # [Output Only] The position of the next byte of content from the serial console output. Use this value in the next request as the start parameter.
   }</pre>
 </div>
 
@@ -971,7 +992,7 @@
         },
         "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
         "diskName": "A String", # Specifies the disk name. If not specified, the default is to use the name of the instance.
-        "sourceImage": "A String", # The source image used to create this disk. If the source image is deleted, this field will not be set.
+        "sourceImage": "A String", # The source image to create this disk. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
             #
             # To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
             #
@@ -988,6 +1009,8 @@
             # You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name:
             #
             # global/images/family/my-private-family
+            #
+            # If the source image is deleted later, this field will not be set.
         "diskType": "A String", # Specifies the disk type to use to create the instance. If not specified, the default is pd-standard, specified using the full URL. For example:
             #
             # https://www.googleapis.com/compute/v1/projects/project/zones/zone/diskTypes/pd-standard
@@ -1006,7 +1029,11 @@
       "mode": "A String", # The mode in which to attach this disk, either READ_WRITE or READ_ONLY. If not specified, the default is to attach the disk in READ_WRITE mode.
       "interface": "A String", # Specifies the disk interface to use for attaching this disk, which is either SCSI or NVME. The default is SCSI. Persistent disks must always use SCSI and the request will fail if you attempt to attach a persistent disk in any other format than SCSI. Local SSDs can use either NVME or SCSI. For performance characteristics of SCSI over NVMe, see Local SSD performance.
       "type": "A String", # Specifies the type of the disk, either SCRATCH or PERSISTENT. If not specified, the default is PERSISTENT.
-      "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. This field is only applicable for persistent disks. Note that for InstanceTemplate, it is just disk name, not URL for the disk.
+      "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
+          #
+          # If desired, you can also attach existing non-root persistent disks using this property. This field is only applicable for persistent disks.
+          #
+          # Note that for InstanceTemplate, specify the disk name, not the URL for the disk.
     },
   ],
   "name": "A String", # The name of the resource, provided by the client when initially creating the resource. The resource name must be 1-63 characters long, and comply with RFC1035. Specifically, the name must be 1-63 characters long and match the regular expression [a-z]([-a-z0-9]*[a-z0-9])? which means the first character must be a lowercase letter, and all following characters must be a dash, lowercase letter, or digit, except the last character, which cannot be a dash.
@@ -1122,7 +1149,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1202,7 +1229,7 @@
               },
               "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
               "diskName": "A String", # Specifies the disk name. If not specified, the default is to use the name of the instance.
-              "sourceImage": "A String", # The source image used to create this disk. If the source image is deleted, this field will not be set.
+              "sourceImage": "A String", # The source image to create this disk. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
                   #
                   # To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
                   #
@@ -1219,6 +1246,8 @@
                   # You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name:
                   #
                   # global/images/family/my-private-family
+                  #
+                  # If the source image is deleted later, this field will not be set.
               "diskType": "A String", # Specifies the disk type to use to create the instance. If not specified, the default is pd-standard, specified using the full URL. For example:
                   #
                   # https://www.googleapis.com/compute/v1/projects/project/zones/zone/diskTypes/pd-standard
@@ -1237,7 +1266,11 @@
             "mode": "A String", # The mode in which to attach this disk, either READ_WRITE or READ_ONLY. If not specified, the default is to attach the disk in READ_WRITE mode.
             "interface": "A String", # Specifies the disk interface to use for attaching this disk, which is either SCSI or NVME. The default is SCSI. Persistent disks must always use SCSI and the request will fail if you attempt to attach a persistent disk in any other format than SCSI. Local SSDs can use either NVME or SCSI. For performance characteristics of SCSI over NVMe, see Local SSD performance.
             "type": "A String", # Specifies the type of the disk, either SCRATCH or PERSISTENT. If not specified, the default is PERSISTENT.
-            "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. This field is only applicable for persistent disks. Note that for InstanceTemplate, it is just disk name, not URL for the disk.
+            "source": "A String", # Specifies a valid partial or full URL to an existing Persistent Disk resource. When creating a new instance, one of initializeParams.sourceImage or disks.source is required.
+                #
+                # If desired, you can also attach existing non-root persistent disks using this property. This field is only applicable for persistent disks.
+                #
+                # Note that for InstanceTemplate, specify the disk name, not the URL for the disk.
           },
         ],
         "name": "A String", # The name of the resource, provided by the client when initially creating the resource. The resource name must be 1-63 characters long, and comply with RFC1035. Specifically, the name must be 1-63 characters long and match the regular expression [a-z]([-a-z0-9]*[a-z0-9])? which means the first character must be a lowercase letter, and all following characters must be a dash, lowercase letter, or digit, except the last character, which cannot be a dash.
@@ -1381,7 +1414,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1442,7 +1475,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1508,7 +1541,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1581,7 +1614,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1649,7 +1682,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1720,7 +1753,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1779,7 +1812,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1857,7 +1890,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>
 
@@ -1916,7 +1949,7 @@
     },
     "endTime": "A String", # [Output Only] The time that this operation was completed. This value is in RFC3339 text format.
     "selfLink": "A String", # [Output Only] Server-defined URL for the resource.
-    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. If creating a persistent disk snapshot, this points to the persistent disk that the snapshot was created from.
+    "targetLink": "A String", # [Output Only] The URL of the resource that the operation modifies. For operations related to creating a snapshot, this points to the persistent disk that the snapshot was created from.
   }</pre>
 </div>