Add documentation about the script and regex linter to the user guide.

Summary:
The big, gigantic comment about the script and regex linter belongs in a more obvious place. I think this is a more obvious place. I also cleaned up a couple things.

I'll update D9084 to remove the big comment block and point here instead.

Test Plan: bin/diviner generate --book src/docs/book/user.book

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9100