Provide documentation about Audit, Differential, and audit vs review

Summary: Try to explain how this stuff works a little better. Let me know what's unclear / missing / not good.

Test Plan: Generated documentation, read documentation.

Reviewers: btrahan, gschmidt

Reviewed By: btrahan

CC: aran, epriestley, davidreuss

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1969