Add more information about Harbormaster/Unit result codes to Arcanist

Summary: Ref T7419. This makes it easier to render helpful documentation in the next diff without having to copy/paste things.

Test Plan:
In next diff:

{F688894}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7419

Differential Revision: https://secure.phabricator.com/D13788