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.
if you set ``columnPrefix="false"``, it will use the ``false`` string as prefix. There is no way to disable the prefix in XML currently. this was forgotten by the guy adding the support of ``false``