Don't trigger "Auto Review" if the author is already an owner; document "Auto Review"

Summary:
Ref T10939. If you already own a package, don't trigger the subscribe/review rules.

Document how these rules work.

Test Plan:

• Read documentation.
• Removed reviewers, updated a revision, got autoreviewed.
• Joined package.
• Removed reveiwers, updated a revision, no more autoreview.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15918