Make the documentation more clear that storage.mysql-engine.max-size is measured in bytes

Summary: Fixes T12001. I think we're consistent about using bytes everywhere, but users won't necessarily know that and this documentation could certainly be more clear.

Test Plan: Read new text.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12001

Differential Revision: https://secure.phabricator.com/D17037