rewrite guide: document REDIRECT_ prefix for [E=] environment variables

rewrite guide: document REDIRECT_ prefix for [E=] environment variables

flags.xml: expand the [E] flag section with a note box explaining
that per-directory rewrites rename env vars with a REDIRECT_ prefix
after internal redirect, with an example showing how to reference the
renamed variable and a note about prefix stacking.

env.xml: add cross-reference from the REDIRECT_ variables section
back to rewrite/flags.html#flag_e for mod_rewrite-specific details.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934203 13f79535-47bb-0310-9956-ffa450edef68