Improve `phd` help

Summary: Fixes T3680. One description was wrong, and clean up some of the other stuff.

Test Plan: Ran `phd`.

Reviewers: btrahan, Korvin

Reviewed By: Korvin

CC: aran, jifriedman, Korvin

Maniphest Tasks: T3680

Differential Revision: https://secure.phabricator.com/D6683