rewrite guide: document mod_rewrite vs mod_alias processing order

rewrite guide: document mod_rewrite vs mod_alias processing order

tech.xml: new "Module Processing Order" section explaining that
mod_rewrite runs before mod_alias in server/vhost context (hook
priority, not config order), and that per-directory context reverses
this. Includes a concrete example and practical guidance.

avoid.xml: add cross-reference note to the "Simple Redirection"
section warning about the order inconsistency.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934198 13f79535-47bb-0310-9956-ffa450edef68