R1:ae0cf00a232f

Document the use of repository commit hints

Summary: Ref T11522. This explains how to actually use `bin/repository hint`.

Test Plan: Read the document. Used `bin/repository hint` as directed.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11522

Differential Revision: https://secure.phabricator.com/D16441