R1:84dcf9045ce2

Clarify mail_handler.php setup instructions

Summary: While not explicitly misleading, this document can do a better job of covering the common/modern case.

Test Plan: Read document.

Reviewers: rush898, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10783