Merge branch '2.8' into 3.4

* 2.8:
  [Form][Validation] User $user instead of $users
  Added 'Default' validation group
  Update Callback.rst

Author: javiereguiluz

form/validation_groups.rst

@@ -10,7 +10,7 @@ Validation Groups
 If your object takes advantage of :doc:`validation groups </validation/groups>`,
 you'll need to specify which validation group(s) your form should use::
 
-    $form = $this->createFormBuilder($users, array(
+    $form = $this->createFormBuilder($user, array(
         'validation_groups' => array('registration'),
     ))->add(...);
 
@@ -28,4 +28,7 @@ method::
     }
 
 In both of these cases, *only* the ``registration`` validation group will
-be used to validate the underlying object.
+be used to validate the underlying object. To apply the ``registration``
+group *and* all constraints that are not in a group, use::
+
+    'validation_groups' => array('Default', 'registration')

reference/constraints/Callback.rst

@@ -272,7 +272,7 @@ Concrete callbacks receive an :class:`Symfony\\Component\\Validator\\Context\\Ex
 instance as only argument.
 
 Static or closure callbacks receive the validated object as the first argument
-and the :class:`Symfony\\Component\\Validator\\ExecutionContextInterface`
+and the :class:`Symfony\\Component\\Validator\\Context\\ExecutionContextInterface`
 instance as the second argument.
 
 .. include:: /reference/constraints/_payload-option.rst.inc