Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/dyn/compute_beta.instanceTemplates.html b/docs/dyn/compute_beta.instanceTemplates.html
index 1212738..9810019 100644
--- a/docs/dyn/compute_beta.instanceTemplates.html
+++ b/docs/dyn/compute_beta.instanceTemplates.html
@@ -147,7 +147,7 @@
     },
     "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>
 
@@ -219,7 +219,7 @@
                   # - projects/project/zones/zone/diskTypes/diskType
                   # - zones/zone/diskTypes/diskType  Note that for InstanceTemplate, this is the name of the disk type, not URL.
               "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
-              "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:
                   #
@@ -236,6 +236,8 @@
                   # 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.
               "diskStorageType": "A String", # [Deprecated] Storage type of the disk.
             },
             "autoDelete": True or False, # Specifies whether the disk will be auto-deleted when the instance is deleted (but not when the disk is detached from the instance).
@@ -247,7 +249,11 @@
             "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.
           },
         ],
         "machineType": "A String", # The machine type to use for instances that are created from this template.
@@ -384,7 +390,7 @@
                 # - projects/project/zones/zone/diskTypes/diskType
                 # - zones/zone/diskTypes/diskType  Note that for InstanceTemplate, this is the name of the disk type, not URL.
             "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
-            "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:
                 #
@@ -401,6 +407,8 @@
                 # 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.
             "diskStorageType": "A String", # [Deprecated] Storage type of the disk.
           },
           "autoDelete": True or False, # Specifies whether the disk will be auto-deleted when the instance is deleted (but not when the disk is detached from the instance).
@@ -412,7 +420,11 @@
           "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.
         },
       ],
       "machineType": "A String", # The machine type to use for instances that are created from this template.
@@ -529,7 +541,7 @@
     },
     "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>
 
@@ -619,7 +631,7 @@
                       # - projects/project/zones/zone/diskTypes/diskType
                       # - zones/zone/diskTypes/diskType  Note that for InstanceTemplate, this is the name of the disk type, not URL.
                   "diskSizeGb": "A String", # Specifies the size of the disk in base-2 GB.
-                  "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:
                       #
@@ -636,6 +648,8 @@
                       # 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.
                   "diskStorageType": "A String", # [Deprecated] Storage type of the disk.
                 },
                 "autoDelete": True or False, # Specifies whether the disk will be auto-deleted when the instance is deleted (but not when the disk is detached from the instance).
@@ -647,7 +661,11 @@
                 "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.
               },
             ],
             "machineType": "A String", # The machine type to use for instances that are created from this template.