Non blocking code documentation is slightly out of date

[travis-leith]

I suspect the final section of https://shiny.posit.co/py/docs/nonblocking.html#executing-on-a-different-threadprocess is no longer applicable thanks to PEP703 and free threading mode availability.

Specifically the discussion around the GIL and the comparison of running with thread pool vs process pool should be updated to reflect that if GIL is disabled then thread pool may be suitable for CPU bound tasks.