Move documentation about X-Herald-Rules to an article

Summary:

• We have a lot of headers now; document them.
• Remove the one random protip from like 3 years ago from all Differential

mail.

Test Plan: generated; read documentation

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T931

Differential Revision: https://secure.phabricator.com/D1748