Merge r720955 from trunk:

Slight clarification to the way rewrite works in per-directory context.
Suggested by Peter Kehl on the users mailing list.

Reviewed by: rbowen, covener, takashi

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@721621 13f79535-47bb-0310-9956-ffa450edef68