R1:bd077bfcb7ac

Update inbound and outbound email documentation

Summary: Fixes T8636. Mention Herald for inbound, update some outbound stuff, do some language / organization tweaks.

Test Plan: Read documentation.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T8636

Differential Revision: https://secure.phabricator.com/D19973