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