Revert "Documentation update."

This reverts commit d0a47d4951b7e5a29d1f8037cce2a290de87c4f3.

The interface parameter is actually used in quite a lot places in PyUSB,
like endpoint checking and interface claiming.

Conflicts:
	docs/tutorial.rst
	usb/core.py
Give a meaningful error in case of an invalid endpoint.

We raise a ValueError exception when the endpoint is not found in the
interface given on write/read methods, with a clear message of what gone
wrong.