Rewrite guide: deduplicate per-directory path stripping explanation

Rewrite guide: deduplicate per-directory path stripping explanation

The same concept (directory prefix is stripped before matching in
per-directory context) was explained in full across three files with
near-identical comparison tables. Now htaccess.xml owns the complete
explanation; intro.xml and tech.xml each have a brief mention with a
cross-reference.

Moved the subrequest/reprocessing detail from tech.xml into
htaccess.xml where it bridges into the looping discussion — most
users will never read the tech doc.

tech.xml retains its unique content about the two API phases
(URL-to-filename hook and Fixup hook).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934108 13f79535-47bb-0310-9956-ffa450edef68