build: run docs regen in synth.py (#1059)

diff --git a/docs/dyn/gmail_v1.users.messages.attachments.html b/docs/dyn/gmail_v1.users.messages.attachments.html
index f2e28c4..15532fb 100644
--- a/docs/dyn/gmail_v1.users.messages.attachments.html
+++ b/docs/dyn/gmail_v1.users.messages.attachments.html
@@ -103,9 +103,9 @@
   An object of the form:
 
     { # The body of a single MIME message part.
-    "data": "A String", # The body data of a MIME message part 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, # Number of bytes for the message part data (encoding notwithstanding).
+    "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.
   }</pre>
 </div>