Document `ThreadPoolExecutor` manual scope propagation

[sentrivana]

Thread pools might reuse threads, which means automatic scope forking won't work correctly without manual steps to isolate the scopes. Document this in the docs.

[linear]

PY-1800 Document `ThreadPoolExecutor` manual scope propagation

[alexander-alderman-webb]

Is this still relevant now merged the threading.ThreadPoolExecutor patch? #4770

[sentrivana]

Would still be nice to mention in the threading integration docs that we have automatic scope propagation when using ThreadPoolExecutor starting from version X.Y 🤔