[3.12] gh-73159 Added clarifications in multiprocessing docs on that objects are pickled. (GH-121686) (#121728)

gh-73159 Added clarifications in multiprocessing docs on that objects are pickled. (GH-121686)

Added explicit comments about that objects are pickled when transmitted via multiprocessing queues and pipes.
(cherry picked from commit b5805892d55e769335c11a994b586355720263ba)

Co-authored-by: Ulrik Södergren <ulrik@digitalfotografen.se>