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