Add missing \versionadded.

Doc/api/concrete.tex

@@ -99,6 +99,7 @@ There is no \cfunction{PyNone_Check()} function for the same reason.
 
 \begin{csimplemacrodesc}{Py_RETURN_NONE}
   Properly handle returning \cdata{Py_None} from within a C function.
+  \versionadded{2.4}
 \end{csimplemacrodesc}