Document that logging registers shutdown as an atexit handler (GH-12378)

Doc/library/logging.rst

@@ -1215,6 +1215,10 @@ functions.
    closing all handlers. This should be called at application exit and no
    further use of the logging system should be made after this call.
 
+   When the logging module is imported, it registers this function as an exit
+   handler (see :mod:`atexit`), so normally there's no need to do that
+   manually.
+
 
 .. function:: setLoggerClass(klass)