docs: improve mod_wsgi documentation (Issue #203)

Based on feedback from Graham Dumpleton.

* Recommend setting global Apache config WSGIRestrictEmbedded On
* Don't set the default value for processes=1 - it has side effects
* Use python-home instead of python-path
* Fix confusing mentioning of root and clarify the use of specifying user and group
* Include WSGIProcessGroup in configuration excerpt