R1:34e85aaeb8b6

Document most of the new Diffusion management panel

Summary: Ref T10923. This isn't complete yet, but reduces lies and increases truths.

Test Plan: Read documentation, clicked new "Documentation" nav item.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15868