Update documentation for deprecated choice_list.

Resources/doc/reference/form_types.rst

@@ -75,8 +75,12 @@ preferred_choices
   defaults to array() - see the `Symfony choice Field Type docs`_ for more info
 
 choice_list
+  **(deprecated in favor of choice_loader since Symfony 2.7)**
   defaults to a ``ModelChoiceList`` built from the other options
 
+choice_loader
+  defaults to a ``ModelChoiceLoader`` built from the other options
+
 model_manager
   defaults to null, but is actually calculated from the linked Admin class.
   You usually should not need to set this manually.