Update documentation to reflect bin/auth changes

Summary: See https://secure.phabricator.com/D18901#249481. Update the docs and a warning string to reflect the new reality that bin/auth recover is now able to recover any account, not just administrators.

Test Plan: Mk 1 eyeball

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20007