Document built-in DOCUMENT_PATH_INFO and USER_NAME that have been available...

Document built-in DOCUMENT_PATH_INFO and USER_NAME that have been available and undocumented for years (already present in the very first commit of mod_include.c in svn history in 1999!)

Tested and works like a charm.

Spotted while looking at PR 34405.

(r1827705 in trunk)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1827716 13f79535-47bb-0310-9956-ffa450edef68