gh-142108: document conversion rules for printf-style formatting

Also add a reference to documentation for old formatting.

Author: skirpichev

Doc/library/stdtypes.rst

@@ -2944,6 +2944,12 @@ The conversion types are:
 |            | character in the result.                            |       |
 +------------+-----------------------------------------------------+-------+
 
+For a general non-string Python object ``obj``, numeric conversion types
+attempt to format values, converted first to built-in :class:`float` (by
+``float(obj)``, for floating-point formats) and :class:`int` (by
+``operator.index(obj)`` for ``'o'``, ``'x'``, ``'X'`` or ``'c'`` formats, or by
+``int(obj)`` for other integer formats) types.
+
 Notes:
 
 (1)

Doc/library/string.rst

@@ -645,7 +645,7 @@ Format examples
 ^^^^^^^^^^^^^^^
 
 This section contains examples of the :meth:`str.format` syntax and
-comparison with the old ``%``-formatting.
+comparison with the old :ref:`printf-style formatting <old-string-formatting>`.
 
 In most of the cases the syntax is similar to the old ``%``-formatting, with the
 addition of the ``{}`` and with ``:`` used instead of ``%``.