pythongh-131912: Use different grouping options for the integral and fractional parts (python#132170)

Author: Prometheus3375

Doc/library/string.rst

@@ -737,10 +737,10 @@ Using the comma or the underscore as a digit group separator::
    '4996_02d2'
    >>> '{:_}'.format(123456789.123456789)
    '123_456_789.12345679'
-   >>> '{:._}'.format(123456789.123456789)
-   '123456789.123_456_79'
-   >>> '{:_._}'.format(123456789.123456789)
-   '123_456_789.123_456_79'
+   >>> '{:.,}'.format(123456789.123456789)
+   '123456789.123,456,79'
+   >>> '{:,._}'.format(123456789.123456789)
+   '123,456,789.123_456_79'
 
 Expressing a percentage::