Fix line lengths in documentation and lint `*.book` files as JSON.

Summary: Fix a few "line too long" lint issues in the Diviner documentation. Also lint `*.book` files as JSON.

Test Plan: Eye-ball it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9695