Clarify "metamta.default-address" instructions and lock the option

Summary: This option no longer needs to be configured if you configure inbound mail (and that's the easiest setup approach in a lot of cases), so stop telling users they have to set it up.

Test Plan: Read documentation and configuration help.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20104