Modernize MetaMTA + daemon documentation

Summary: MetaMTA + daemons used to be pretty hard but @nh landed some patches a while ago that make it way eaiser. Back off the "ooh scary config" text in the documentation, since this option will just work for ~every install now.

Test Plan: Read it.

Reviewers: btrahan, vrana, nh

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1525

Differential Revision: https://secure.phabricator.com/D3037