add versionadded for the data collector form extension

Author: xabbuh

components/form/form_events.rst

@@ -110,6 +110,9 @@ the form.
 
 .. sidebar:: ``FormEvents::POST_SET_DATA`` in the Form component
 
+    .. versionadded:: 2.4
+        The data collector extension was introduced in Symfony 2.4.
+
     The :class:`Symfony\\Component\\Form\\Extension\\DataCollector\\EventListener\\DataCollectorListener`
     class is subscribed to listen to the ``FormEvents::POST_SET_DATA`` event
     in order to collect information about the forms from the denormalized
@@ -218,6 +221,9 @@ It can be used to fetch data after denormalization.
 
 .. sidebar:: ``FormEvents::POST_SUBMIT`` in the Form component
 
+    .. versionadded:: 2.4
+        The data collector extension was introduced in Symfony 2.4.
+
     The :class:`Symfony\\Component\\Form\\Extension\\DataCollector\\EventListener\\DataCollectorListener`
     subscribes to the ``FormEvents::POST_SUBMIT`` event in order to collect
     information about the forms.