Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/dyn/gmail_v1.users.messages.attachments.html b/docs/dyn/gmail_v1.users.messages.attachments.html
index 0e040b2..a5ea7fb 100644
--- a/docs/dyn/gmail_v1.users.messages.attachments.html
+++ b/docs/dyn/gmail_v1.users.messages.attachments.html
@@ -91,9 +91,9 @@
   An object of the form:
 
     { # The body of a single MIME message part.
-    "data": "A String", # The body data of a MIME message part. May be empty for MIME container types that have no message body or when the body data is sent as a separate attachment. An attachment ID is present if the body data is contained in a separate attachment.
+    "data": "A String", # The body data of a MIME message part as a base64url encoded string. May be empty for MIME container types that have no message body or when the body data is sent as a separate attachment. An attachment ID is present if the body data is contained in a separate attachment.
     "attachmentId": "A String", # When present, contains the ID of an external attachment that can be retrieved in a separate messages.attachments.get request. When not present, the entire content of the message part body is contained in the data field.
-    "size": 42, # Total number of bytes in the body of the message part.
+    "size": 42, # Number of bytes for the message part data (encoding notwithstanding).
   }</pre>
 </div>