Documentation

I've highlighted the error in bold in the following passage from the doc.

When add_argument() is called with option strings (like -f or --foo) and nargs='?'. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value of const will be assumed to be None instead.

The bold text should be replaced with "used".

Justification: the previous passage is contradicted by the following passage from the same page.

for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from const will be produced.