gh-132898: Add a note in `multiprocessing.Process` docs about creating a process in a REPL (GH-137118)

* Reword, expand, and clarify the limitation, highlighting the REPL case.
* Mention in the high level Process description.
* added a pointer to the GH issue from the doc note

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Co-authored-by: Gregory P. Smith <greg@krypto.org>