Add a section to mpm-event's documentation to advertise new changes

I tried to add a summary of Stefan's last patches just backported to
2.4.x today. I've read all of them and tried to report Stefan's comment
from PR 53555 as much as possible. Please review and let me know if I wrong
something incorrect or not precise enough.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1772357 13f79535-47bb-0310-9956-ffa450edef68