ap_sub_req_lookup_dirent() depends on the over-allocation done by

ap_make_full_path and ap_escape_uri, so let's document it so that it is not
accidentally removed.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1384924 13f79535-47bb-0310-9956-ffa450edef68