Update Google auth documentation to discuss "Google+ API" and new console URI

Summary: Fixes T4451. See also D8612.

Test Plan: Viewed panel and read text, saw it matched up with the new console.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4451

Differential Revision: https://secure.phabricator.com/D8613