A little more general MPM documentation.

Feeback welcome.
Note the comment at the bottom:
<!-- XXX: Needed here: a brief discussion or reference to instructions
on how to choose and load an MPM, how to figure out what MPM you are
currently using, and what MPMs are the defaults on various
platforms. -->


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87060 13f79535-47bb-0310-9956-ffa450edef68