bpo-34415: Updated logging.Formatter docstring. (GH-8811) (GH-8816)

(cherry picked from commit d3d3171)

Lib/logging/__init__.py

@@ -431,7 +431,8 @@ class Formatter(object):
     responsible for converting a LogRecord to (usually) a string which can
     be interpreted by either a human or an external system. The base Formatter
     allows a formatting string to be specified. If none is supplied, the
-    default value of "%s(message)" is used.
+    the style-dependent default value, "%(message)s", "{message}", or
+    "${message}", is used.
 
     The Formatter can be initialized with a format string which makes use of
     knowledge of the LogRecord attributes - e.g. the default value mentioned