Document passing PyClass values as arguments

[davidhewitt]

Just thinking it might be worth adding a table to the guide (class.md?) which talks about the different choices for taking #[pyclass] values as arguments.

Currently there's MyClass, &MyClass, &self, &mut self, PyRef<MyClass>, PyRefMut<MyClass>, and &PyCell<MyClass>.

All have moments when you might want them, but it would help users if this was documented better!