Update Owners auditing rules for multiple reviewers

Summary:
Ref T10939. Fixes T10181. This slightly simplifies, then documents the auditing rules, which haven't been updated for a while. In particular:

• If an owner authored the change, never audit.
• Examine all reviewers to determine reviewer audit status, not just the first reviewer.
• Simplify some of the loading code a bit.

Test Plan:

• Ran bin/repository reparse --owners <commit> --force to trigger this stuff.
• Verified that the web UI did reasonable things with resulting audits.
• Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10181, T10939

Differential Revision: https://secure.phabricator.com/D15939