R1:3f36e498b7b3

Document the new watcher/member/edit notification mail rules

Summary: Ref T10054. This is mostly for completness so I can reference it when closing all the related tasks.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15066