Added description for the "validation_groups" option

As requested in #4401 a description of the "validation_groups" option of the base ``FormType`` is added.

Author: naitsirch

reference/forms/types/form.rst

@@ -34,6 +34,7 @@ on all types for which ``FormType`` is the parent.
 |           | - `read_only`_ (deprecated as of 2.8)                              |
 |           | - `required`_                                                      |
 |           | - `trim`_                                                          |
+|           | - `validation_groups`_                                             |
 +-----------+--------------------------------------------------------------------+
 | Inherited | - `attr`_                                                          |
 | options   | - `auto_initialize`_                                               |
@@ -146,6 +147,8 @@ The actual default value of this option depends on other field options:
 
 .. include:: /reference/forms/types/options/trim.rst.inc
 
+.. include:: /reference/forms/types/options/validation_groups.rst.inc
+
 Inherited Options
 -----------------
 

reference/forms/types/options/validation_groups.rst.inc

@@ -0,0 +1,51 @@
+validation_groups
+~~~~~~~~~~~~~~~~~
+
+**type**: ``array``, ``string``, ``callable`` or ``null`` **default**: ``null``
+
+This option is only valid on the root form and is used to specify which
+groups will be used by the validator.
+
+For ``null`` the validator will just use the ``Default`` group.
+
+If you specify the groups as array or string they will used by the validator
+as they are::
+
+    public function configureOptions(OptionsResolver $resolver)
+    {
+        $resolver->setDefaults(array(
+            'validation_groups' => 'Registration',
+        ));
+    }
+
+This is equivalent to passing the group as array::
+
+    'validation_groups' => array('Registration'),
+
+If the validation groups depend on the form's data a callable may be passed to
+the option. Symfony then will pass the form when calling it::
+
+    use Symfony\Component\Form\FormInterface;
+    
+    // ...
+    public function configureOptions(OptionsResolver $resolver)
+    {
+        $resolver->setDefaults(array(
+            'validation_groups' => function (FormInterface $form) {
+                $entity = $form->getData();
+                return $entity->getValidationGroups();
+            },
+        ));
+    }
+
+The class that the form is used for may look like this::
+
+    class Account
+    {
+        public function getValidationGroups()
+        {
+            return $this->isUser() ? array('User') : array('Company');
+        }
+    }
+
+You can read more about this in :doc:`/form/data_based_validation`.