Document recommendation to have GPU on scheduler if workers have GPU

In the discussion on https://github.com/dask/distributed/pull/6908 it emerged that it's recommended to have a GPU on the scheduler if you're using GPU workers.

As far as I can tell, that's not documented anywhere. It would be super if there was also some explanation of why this is recommended.

I don't know enough to write that so tagging @jacobtomlinson @pentschev @quasiben because they're the people who said this was best practice.