Shorten the comment for EnableMMap, and limit it just to the NFS issue.

The most we could say about preformance is "it's usually better to leave this
on, but sometimes it's better to turn it off", which is not really helpful
to anyone.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95191 13f79535-47bb-0310-9956-ffa450edef68