Add the perchild MPM docs and do a little bit of cleanup.

This finishes up the basic structure of the windows and unix
MPMs.  I can do a little more cleanup, but this will need some heavy
input from others, because I don't feel like going through each
MPM to figure out what all these directive actually do.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87029 13f79535-47bb-0310-9956-ffa450edef68