1009429 - Document the selinux boolean.

https://bugzilla.redhat.com/show_bug.cgi?id=1009429
I feel a little on the fence about this. We always talk about Apache in our docs, but there's not necessarily any reason a pulp_puppet user needs to deploy Pulp with httpd as opposed to NGinx as far as I know.

In fact, I've been meaning to make sure my pulp_puppet plugin + Pulp will work with Gunicorn, so people can deploy it in pure Python.