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.
Documented this bug at http://www.doctrine-project.org/jira/browse/DDC-3293