Document PyUSB's find() arguments in usbreq.find()

Brought up here https://github.com/Qyriad/python-usbreq/issues/1#issuecomment-1703701984: PyUSB's `usb.core.find()`'s arguments aren't super well documented, and even if they were we should explain how to use them in `usbreq.find()`'s docstring.