Documentation for kwargs in instance method #236

As far as I've been able to determine one needs to specify *args for **kwargs to work in an instance method. The documentation for py_argparse indicates that this is a bug. It allows parameter to have either syntax but doesn't specify that one is required for the other to work. Python also allows it (e.g. def f(**kwargs): is a valid signature).

// Doesn't work :(
// [Error] no rules expected the token `{`
py_class!(class Example |py| {
    def _replace(&self, **kwargs) -> PyResult<NoArgs> {
        Ok(NoArgs)
    }
});

// Workaround that works :)
py_class!(class Example |py| {
    def _replace(&self, *args, **kwargs) -> PyResult<NoArgs> {
        Ok(NoArgs)
    }
});

If it significantly affects the complexity of the implementation then this restriction is fine and the workaround definitely works for my use case. Feel free to regard this as a pure documentation bug if you want.