Adds a sentence about Compute API v2 and v3 XML deprecation

 - for requests and responses in the Compute APIs

Also, use clouddocs-maven-plugin 2.1.2.

Change-Id: I1cd2e04e4634180141e67c277a6d9607ecfb23d9
Partial-Bug:1283341
Merge "Adds a sentence about Compute API v2 and v3 XML deprecation"