MetaMTA - update documentation and make config a tad easier

Summary: Fixes T7088. Mainly this updates the documentation but I also snuck in tweaking how the domain reply handler is built. This does two main things -- makes the behavior consistent as some applications who didn't override this behavior would send out emails with reply tos AND makes it easier for us to deprecate the custom domain thing on a per application basis, which is just silly. On that note, the main documentation doesn't get into how this can be overridden, though I left in that mini blurb on the config setting itself. We could deprecate this harder and LOCK things if you want as well.

Test Plan: read docs, looked good. reasoned through re-factor

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7088

Differential Revision: https://secure.phabricator.com/D11725