describe the allow_extra_fields form option

book/forms.rst

@@ -625,6 +625,20 @@ 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 Extra Fields
+~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 2.6
+    The ``allow_extra_fields`` option was introduced in Symfony 2.6.
+
+Usually, if you submit extra fields that aren't configured in your form,
+you'll get a "This form should not contain extra fields." validation error.
+
+You can silence this validation error 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