Generate mail command documentation from active commands

Summary: Ref T7199. This needs some polish and isn't reachable from the UI, but technically has all of the information.

Test Plan:
{F355899}

{F355900}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12241