describe the allow_extra_fields form option

Author: xabbuh

book/forms.rst

@@ -625,6 +625,21 @@ Now the form will skip your validation constraints. It will still validate
 basic integrity constraints, such as checking whether an uploaded file was too
 large or whether you tried to submit text in a number field.
 
+Handling Additional Fields
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 2.6
+    The ``allow_extra_fields`` option was introduced in Symfony 2.6.
+
+Usually, the Form component will complain when your submitted form contains
+fields that were not configured before (for example, when fields are added
+using JavaScript or when a page contains several forms without a name).
+
+You can silence this error message by enabling the ``allow_extra_fields``
+option on the form::
+
+    $form = $this->createForm(new TaskType(), $task, array('allow_extra_fields' => true));
+
 .. index::
    single: Forms; Built-in field types