gh-84559: improve What's New entry for multiprocessing start method changes

[picnixz]

This is a proposal for an improved changelog. I am tempted to add it at the very top of the file in the "Summary -- release highlights" but I don't know if @hugovk is ok or not with it.

• Issue: multiprocessing's default posix start method of 'fork' is broken: change to `'forkserver' || 'spawn' #84559

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📚 Documentation preview 📚: https://cpython-previews--128173.org.readthedocs.build/

[hugovk]

I am tempted to add it at the very top of the file in the "Summary -- release highlights" but I don't know if @hugovk is ok or not with it.

Yeah, this sounds like a big change we want people to know about, let's add a summary up there and link down to multiprocessing part.

I expect we'll expand the summary closer to the release, compare 3.13:

https://docs.python.org/3/whatsnew/3.13.html#summary-release-highlights

cc @AA-Turner

[jakirkham]

Just a note, on macOS the default is spawn (as it has been for a while). However the docs also note that POSIX uses forkserver by default. Though macOS also is a POSIX platform. Should this be disambiguated somehow?

[picnixz]

Though macOS also is a POSIX platform. Should this be disambiguated somehow

Ah yes, sorry. it should have been "On POSIX platforms, except macOS, [...]" or "On platforms other than macOS and Windows". Which one would be the clearer?