Update lint engine documentation

Summary: Fixes T6627. This class documentation is out of date and somewhat misleading in the modern environment.

Test Plan: Read it.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6627

Differential Revision: https://secure.phabricator.com/D10899
Merge branch 'master' of github.com:facebook/arcanist