Better document techniques for reusing linters

Summary: As per discussion with @johnduhart, improve documentation around
reusing and customizing linters.

Test Plan: Generated and read documentation.

Reviewers: btrahan, jungejason, johnduhart

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T795

Differential Revision: https://secure.phabricator.com/D1501