[3.9] [doc] Document logging.basicConfig default format (GH-23710) (GH-24835)

(cherry picked from commit 35cacce)

Author: miss-islington

Doc/library/logging.rst

@@ -1176,7 +1176,9 @@ functions.
    |              | to ``'a'``.                                 |
    +--------------+---------------------------------------------+
    | *format*     | Use the specified format string for the     |
-   |              | handler.                                    |
+   |              | handler. Defaults to attributes             |
+   |              | ``levelname``, ``name`` and ``message``     |
+   |              | separated by colons.                        |
    +--------------+---------------------------------------------+
    | *datefmt*    | Use the specified date/time format, as      |
    |              | accepted by :func:`time.strftime`.          |