Improve help for "bin/policy unlock" to make it explicit that you can name objects with a PHID

Summary: See https://discourse.phabricator-community.org/t/how-to-unlock-a-portal-to-view-edit-it-as-an-admin-similar-to-tasks/4547/.

Test Plan: Ran bin/policy help unlock, saw explicit "you can use a PHID" documentation.

Differential Revision: https://secure.phabricator.com/D21530