Add references to locale module for locale-aware number formatting

Author: stefmolin

Doc/library/string.rst

@@ -469,7 +469,9 @@ of a number respectively. It can be one of the following:
 |         | this option is not supported.                            |
 +---------+----------------------------------------------------------+
 
-For a locale aware separator, use the ``'n'`` presentation type instead.
+For a locale-aware separator, use the ``'n'`` presentation type instead. Note
+that the locale setting for numeric values must first be set using the
+:mod:`locale` module, *e.g.,* ``locale.setlocale(locale.LC_NUMERIC, 'en_US')``.
 
 .. versionchanged:: 3.1
    Added the ``','`` option (see also :pep:`378`).
@@ -517,7 +519,8 @@ The available integer presentation types are:
    +---------+----------------------------------------------------------+
    | ``'n'`` | Number. This is the same as ``'d'``, except that it uses |
    |         | the current locale setting to insert the appropriate     |
-   |         | digit group separators.                                  |
+   |         | digit group separators, provided that the locale has     |
+   |         | been properly set (see the :mod:`locale` module).        |
    +---------+----------------------------------------------------------+
    | None    | The same as ``'d'``.                                     |
    +---------+----------------------------------------------------------+
@@ -599,8 +602,9 @@ The available presentation types for :class:`float` and
    +---------+----------------------------------------------------------+
    | ``'n'`` | Number. This is the same as ``'g'``, except that it uses |
    |         | the current locale setting to insert the appropriate     |
-   |         | digit group separators                                   |
-   |         | for the integral part of a number.                       |
+   |         | digit group separators for the integral part of a        |
+   |         | number, provided that the locale has been properly set   |
+   |         | (see the :mod:`locale` module).                          |
    +---------+----------------------------------------------------------+
    | ``'%'`` | Percentage. Multiplies the number by 100 and displays    |
    |         | in fixed (``'f'``) format, followed by a percent sign.   |