bpo-45735: Promise the long-time truth that `args=list` works (GH-30982)

For threads, and for multiprocessing, it's always been the case that ``args=list`` works fine when passed to ``Process()`` or ``Thread()``, and such code is common in the wild. But, according to the docs, only a tuple can be used. This brings the docs into synch with reality.

Doc changes by Charlie Zhao.
Co-authored-by: Tim Peters <tim.peters@gmail.com>