[#6958] add missing versionadded directive

Author: xabbuh

form/create_form_type_extension.rst

@@ -136,6 +136,9 @@ field type, you will use that as the ``extended_type``.
     an extension is loaded). This is useful when you need to guarantee 
     that one extension is loaded before or after another extension.
 
+    .. versionadded:: 3.2
+        The ``priority`` attribute was introduced in Symfony 3.2.
+
 Adding the extension Business Logic
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