Issue #17728: Specify default precision for float.format for presentation types e, f, and g.

Doc/library/string.rst

@@ -432,12 +432,13 @@ The available presentation types for floating point and decimal values are:
    +=========+==========================================================+
    | ``'e'`` | Exponent notation. Prints the number in scientific       |
    |         | notation using the letter 'e' to indicate the exponent.  |
+   |         | The default precision is ``6``.                          |
    +---------+----------------------------------------------------------+
    | ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an     |
    |         | upper case 'E' as the separator character.               |
    +---------+----------------------------------------------------------+
    | ``'f'`` | Fixed point. Displays the number as a fixed-point        |
-   |         | number.                                                  |
+   |         | number.  The default precision is ``6``.                 |
    +---------+----------------------------------------------------------+
    | ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to    |
    |         | ``NAN`` and ``inf`` to ``INF``.                          |
@@ -464,7 +465,7 @@ The available presentation types for floating point and decimal values are:
    |         | the precision.                                           |
    |         |                                                          |
    |         | A precision of ``0`` is treated as equivalent to a       |
-   |         | precision of ``1``.                                      |
+   |         | precision of ``1``.  The default precision is ``6``.     |
    +---------+----------------------------------------------------------+
    | ``'G'`` | General format. Same as ``'g'`` except switches to       |
    |         | ``'E'`` if the number gets too large. The                |