clarified usage of the dump function (closes #718)

doc/functions/dump.rst

@@ -14,13 +14,19 @@ introspecting its variables:
 
 .. note::
 
-    The ``debug`` function is not available by default. You must load it explicitly::
+    The ``dump`` function is not available by default. You must add the
+    ``Twig_Extension_Debug`` extension explicitly when creating your Twig
+    environment::
 
-        $twig = new Twig_Environment($loader, $config);
+        $twig = new Twig_Environment($loader, array(
+            'debug' => true,
+            // ...
+        ));
         $twig->addExtension(new Twig_Extension_Debug());
 
-    Even when loaded explicitly, it won't do anything if the ``debug`` option
-    is not enabled.
+    Even when enabled, the ``dump`` function won't display anything if the
+    ``debug`` option on the environment is not enabled (to avoid leaking debug
+    information on a production server).
 
 In an HTML context, wrap the output with a ``pre`` tag to make it easier to
 read:
@@ -54,5 +60,5 @@ dumped:
 
     Internally, Twig uses the PHP `var_dump`_ function.
 
-.. _`XDebug`: http://xdebug.org/docs/display
+.. _`XDebug`:   http://xdebug.org/docs/display
 .. _`var_dump`: http://php.net/var_dump