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
Support HTTP basic auth in HTTPFuture

Summary: Ref T4590. We already sort of support this for Conduit, push the support a little bit deeper.

Test Plan: Added basic auth across my entire nginx. Requests with no auth or with the wrong auth failed; requests with the right auth worked.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4590

Differential Revision: https://secure.phabricator.com/D8621