document the parent_form() Twig function

Author: xabbuh

form/form_customization.rst

@@ -312,6 +312,18 @@ obvious (since it'll render the field for you).
 
     {{ form_rest(form) }}
 
+parent_form(form_view)
+......................
+
+.. versionadded:: 4.3
+
+    The ``parent_form()`` function was introduced in Symfony 4.3.
+
+Returns the parent form view or ``null`` if the form view already is the
+root form. Using this function should be preferred over accessing the parent
+form using ``form.parent``. The latter way will produce different results
+when a child form is named ``parent``.
+
 Tests
 ~~~~~