Add docs and facilities for having separate directories of schemas.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

library/HTMLPurifier/Config.php

@@ -216,6 +216,7 @@ class HTMLPurifier_Config
 
     /**
      * Retrieves all directives, organized by namespace
+     * @warning This is a pretty inefficient function, avoid if you can
      */
     public function getAll() {
         if (!$this->finalized) $this->autoFinalize();