gh-123560: Correct docs for "empty" format type for floats (#123561)

Author: skirpichev

Doc/library/string.rst

@@ -574,11 +574,13 @@ The available presentation types for :class:`float` and
    | ``'%'`` | Percentage. Multiplies the number by 100 and displays    |
    |         | in fixed (``'f'``) format, followed by a percent sign.   |
    +---------+----------------------------------------------------------+
-   | None    | For :class:`float` this is the same as ``'g'``, except   |
+   | None    | For :class:`float` this is like the ``'g'`` type, except |
    |         | that when fixed-point notation is used to format the     |
    |         | result, it always includes at least one digit past the   |
-   |         | decimal point. The precision used is as large as needed  |
-   |         | to represent the given value faithfully.                 |
+   |         | decimal point, and switches to the scientific notation   |
+   |         | when ``exp >= p - 1``.  When the precision is not        |
+   |         | specified, the latter will be as large as needed to      |
+   |         | represent the given value faithfully.                    |
    |         |                                                          |
    |         | For :class:`~decimal.Decimal`, this is the same as       |
    |         | either ``'g'`` or ``'G'`` depending on the value of      |