bpo-38678: Improve argparse example in tutorial (GH-17207)

diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index 9a1cf3a..f8e3918 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -810,10 +810,12 @@
   example, this is useful for increasing verbosity levels::
 
     >>> parser = argparse.ArgumentParser()
-    >>> parser.add_argument('--verbose', '-v', action='count')
+    >>> parser.add_argument('--verbose', '-v', action='count', default=0)
     >>> parser.parse_args(['-vvv'])
     Namespace(verbose=3)
 
+  Note, the *default* will be ``None`` unless explicitly set to *0*.
+
 * ``'help'`` - This prints a complete help message for all the options in the
   current parser and then exits. By default a help action is automatically
   added to the parser. See :class:`ArgumentParser` for details of how the