Update Doc/library/string.rst

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: skirpichev

Doc/library/string.rst

@@ -411,6 +411,7 @@ conversions, trailing zeros are not removed from the result.
 
 The ``','`` option signals the use of a comma for a thousands separator for
 floating-point presentation types and for integer presentation type ``'d'``.
+For other presentation types, this option is an error.
 For a locale aware separator, use the ``'n'`` integer presentation type
 instead.