make a note about an issue listed as a showstopper

even with no change, user can --disable-threads so I wonder if
this should really be a showstopper; it would be nice to get
feedback from the user on my patch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95491 13f79535-47bb-0310-9956-ffa450edef68