Improve instructions for Google auth mess

Summary:
See T4676 for context. @csilvers suggested that these changes would have been helpful in resolving this issue.

If we get like one more report I'll probably try to back this out and phase it in more gradually instead, although we're going to hit something like this on Sept 1 no matter what.

Test Plan:
Faked it.

{F135327}

Reviewers: btrahan, csilvers

Reviewed By: csilvers

Subscribers: csilvers, epriestley

Differential Revision: https://secure.phabricator.com/D8625