backport r1379321: Note that using rewrites with a file-path substitution in a...

backport r1379321: Note that using rewrites with a file-path substitution in a .htaccess file will look for files relative to your document root, not relative to your file system root.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1379701 13f79535-47bb-0310-9956-ffa450edef68