Add event in mpm docs where currently only

worker is mentioned. Existing worker text
applies to event without changes.

Thanks to Mathijs Schmittmann.

CTR
Backport of r1707626 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1707631 13f79535-47bb-0310-9956-ffa450edef68