Provide more help text for `arc linters`

Summary: Ref T2039. This isn't exhaustive, but moves things forward by a decent chunk.

Test Plan: Used `arc linters` and read the messages.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T2039

Differential Revision: https://secure.phabricator.com/D9056