D417: Ignore arguments marked as unused

[karlch]

First of all thanks for a really useful tool and the great D417 which has caught many issues for me!

I feel like D417 should not complain for arguments marked as unused by prepending an underscore. This is also how pylint handles the unused-argument error.

Consider e.g.:

def handle_interrupt(self, signum: int, _frame: FrameType) -> None:
    ...

We provide the _frame argument to make the callback signature valid, but never actually use it in the later code. IMHO documenting _frame in this case should not be required.