minor #3887 [Components][Form] add versionadded for the data collecto…

…r form extension (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
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[Components][Form] add versionadded for the data collector form extension

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

As @ahsio pointed out in #3886, the data collector extension was added to the Form component in Symfony 2.4.

Commits
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beceed4 add versionadded for the data collector form extension

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.