Don't recommend putting Phabricator on the main Directory directive

Summary: Fixes T5428. The rules are different under <Directory /> and this is very rare, so don't make this harder than it needs to be.

Test Plan: Read text.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5428

Differential Revision: https://secure.phabricator.com/D9646