gh-127296: Document that argparse count action starts from a non-zero default

Author: owenthcarey

Doc/library/argparse.rst

@@ -836,6 +836,8 @@ how the command-line arguments should be handled. The supplied actions are:
     Namespace(verbose=3)
 
   Note, the *default* will be ``None`` unless explicitly set to *0*.
+  If the default value is a non-zero number, the count starts from that
+  number rather than from zero.
 
 * ``'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