Consolidate per-directory rewrite notes: remove duplication between mod_rewrite.xml...

Consolidate per-directory rewrite notes: remove duplication between mod_rewrite.xml and rewrite/htaccess.xml

The large "Per-directory Rewrites" note block in mod_rewrite.xml
duplicated content already covered (and better explained) in
rewrite/htaccess.xml. Replace it with a short link to the guide.

Migrate the useful tips that were unique to that block into
rewrite/htaccess.xml:
- ^/ never matches in per-directory context (path-stripping section)
- Use %{REQUEST_URI} to match the full URL-path, with example
- <Location>/<Files> are unsupported contexts (new section)
- <If> follows directory context rules (new section)

Part of: https://bz.apache.org/bugzilla/show_bug.cgi?id=70045

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934228 13f79535-47bb-0310-9956-ffa450edef68