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