flags.xml: Clarify DPI flag docs, replace "URI" with "URL-path", add example

flags.xml: Clarify DPI flag docs, replace "URI" with "URL-path", add example

In RFC terms, URI and URL are functionally interchangeable on the web
(all HTTP URLs are URIs; RFC 3986 subsumes both). However, in this
context the docs mean specifically the path component — no scheme, no
host, no port. "URL-path" (as defined in our own directive-dict.xml)
makes this unambiguous, and is consistent with the rest of the
mod_rewrite documentation which uses "URL-path" throughout.

Also add a concrete example showing DPI preventing PATH_INFO
duplication when rewriting a request.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934232 13f79535-47bb-0310-9956-ffa450edef68