Use ap_unescape_url_keep2f() in ap_expr unescape func. ap_unescape_url()

forbidding encoded slashes is not useful here.
Log failures.
Improve docs.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1151373 13f79535-47bb-0310-9956-ffa450edef68