Improve documentation and tooling around autoclose

Summary:
Fixes T4767. I believe 80% of this was actually caused by the author issue fixed in T5771, but this should help make the other 20% debuggable.

  - Record why we didn't autoclose a commit when we process it.
  - Show branch autoclose status in the main branch table.
  - Show commit autoclose status on the edit screen.
  - Add documentation about how to find these statuses and what they mean.

Test Plan:
  - Read documentation.
  - Viewed branches and hovered over the various states.
  - Viewed commits in various states and checked the "Autoclose?" field.
  - Pushed some commits and saw autoclose activate.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4767

Differential Revision: https://secure.phabricator.com/D10348