Added a reference to the mpm-event's doc about the fact that it does not...

Added a reference to the mpm-event's doc about the fact that it does not require any mpm-accept mutex set.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1741733 13f79535-47bb-0310-9956-ffa450edef68