Added some minor explanation about 308 code

Author: javiereguiluz

routing/redirect_trailing_slash.rst

@@ -10,7 +10,7 @@ trailing slash to the same URL without a trailing slash
 
 Create a controller that will match any URL with a trailing slash, remove
 the trailing slash (keeping query parameters if any) and redirect to the
-new URL with a 308 response status code::
+new URL with a 308 (*HTTP Permanent Redirect*) response status code::
 
     // src/AppBundle/Controller/RedirectingController.php
     namespace AppBundle\Controller;
@@ -27,6 +27,8 @@ new URL with a 308 response status code::
 
             $url = str_replace($pathInfo, rtrim($pathInfo, ' /'), $requestUri);
 
+            // 308 (Permanent Redirect) is similar to 301 (Moved Permanently) except
+            // that it does not allow changing the request method from POST to GET
             return $this->redirect($url, 308);
         }
     }