Document embeddables column prefixing

Motivation: https://groups.google.com/forum/#!topic/doctrine-user/xDiL65QV_sM

Documents column prefixing for embeddables. Adds headings.

I can't use XML or YAML configuration, so I'm not sure if those are correct.
Well, ignoring the property being wrong, I put `columnPrefix="false"`, which would make it a string, right? If not, what would be the correct declaration?