Clarified variable passing in recursive form rendering.

See symfony/symfony#5565

reference/forms/twig_reference.rst

@@ -49,7 +49,9 @@ or collection of fields, each underlying form row will be rendered.
 The second argument to ``form_widget`` is an array of variables. The most
 common variable is ``attr``, which is an array of HTML attributes to apply
 to the HTML widget. In some cases, certain types also have other template-related
-options that can be passed. These are discussed on a type-by-type basis.
+options that can be passed. These are discussed on a type-by-type basis. Note 
+that in case the first argument is a form, all ``variables`` will 
+only be applied to the form itself and not its children.
 
 form_row(form.name, variables)
 ------------------------------