bpo-33828: Add missing versionchanged note for string.Formatter. (GH-7668)

string.Formatter auto-numbering feature was added in 3.4 and there
is no versionchanged note in its documentation, making the documentation
ambiguous about which version the feature is available.
(cherry picked from commit b9d8ad5)

Co-authored-by: Xiang Zhang <[email protected]>

Author: miss-islington

Doc/library/string.rst

@@ -231,8 +231,11 @@ attribute using :func:`getattr`, while an expression of the form ``'[index]'``
 does an index lookup using :func:`__getitem__`.
 
 .. versionchanged:: 3.1
-   The positional argument specifiers can be omitted, so ``'{} {}'`` is
-   equivalent to ``'{0} {1}'``.
+   The positional argument specifiers can be omitted for :meth:`str.format`,
+   so ``'{} {}'.format(a, b)`` is equivalent to ``'{0} {1}'.format(a, b)``.
+
+.. versionchanged:: 3.4
+   The positional argument specifiers can be omitted for :class:`Formatter`.
 
 Some simple format string examples::