Fix MaxClients description to adapt threading MPM.

(This is still inaccurate for trunk Event MPM.)

Submitted by Dan Poirier <poirier pobox.com>
PR: 45747

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@692325 13f79535-47bb-0310-9956-ffa450edef68