Make it clear what is meant by per-directory rewrites

and specify the rewrites in <Location> is unsupported.
(The same is probably true for <Files>, but I'd have to
think about that a little more.)

PR: 39843


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@418360 13f79535-47bb-0310-9956-ffa450edef68