Fill in missing cluster database documentation

Summary:
Ref T10751. Provide some guidance on replicas and promotion.

I'm not trying to walk administrators through the gritty details of this. It's not too complex, they should understand it, and the MySQL documentation is pretty thorough.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15763