Although for me it looks clear and straightforward, for some it may not be the case, so I decided to add a brief document explaining it. It should show up in Read The Docs here: http://jenkinsploneorg.readthedocs.org/en/latest/run-pull-request-jobs.html
If Read The Docs still hasn’t catch up the source in GitHub is easily readable as well: https://github.com/plone/jenkins.plone.org/blob/master/docs/source/run-pull-request-jobs.rst
Bonus point: I made my first screencast demoing it! Watch it in all its glory.