Update Owners docs a bit

Summary:
Fixes T9218. Fixes T8320. Fixes T8661. This isn't exhaustive but documents the stuff that cropped up in this iteration as needing documentation. In particular:

• Be explicit about multiple ownership.
• Explain value of having one place to update your giant regexp of a trillion paths.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8320, T8661, T9218

Differential Revision: https://secure.phabricator.com/D14023