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