Document git:branch-unique(*) and arc:bookmark
Summary: Explain how these work. Let me know if there's a clearer way to explain "arc:bookmark".
Test Plan: Generated / viewed documentation.
Reviewers: dschleimer
Reviewed By: dschleimer
CC: aran
Maniphest Tasks: T1233
Differential Revision: https://secure.phabricator.com/D2868