Document the adjustment workflow and warn users about adjusting old MySQL

Summary: Ref T1191. Explain the adjustment workflow, how to resolve common errors, etc.

Test Plan: Read it, clicked doc links.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10758