describe the allow_extra_fields form option

Author: xabbuh

book/forms.rst

@@ -1084,6 +1084,15 @@ the choice is ultimately up to you.
                 ->add('save', 'submit');
         }
 
+    .. versionadded:: 2.6
+        The ``allow_extra_fields`` option was introduced in Symfony 2.6.
+
+    If you have many additional fields, this task may become tedious. Since Symfony
+    2.6, you can use the ``allow_extra_fields`` option when building the form
+    to ignore all extra fields::
+
+        $form = $this->createForm(new TaskType(), $task, array('allow_extra_fields' => true));
+
     Additionally, if there are any fields on the form that aren't included in
     the submitted data, those fields will be explicitly set to ``null``.