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.
Instead of putting comments in the code snippet, It would be better to have smaller snippets and the explanation outside the code. This would also avoid duplicating the explanation for each format