[3.13] gh-84559: improve What's New entry for `multiprocessing` startmethod changes (GH-128173) (#137156)

[3.13] gh-84559: improve What's New entry for `multiprocessing` start method changes (GH-128173)
(cherry picked from commit b8c313a41c411331c603bbda28048b2a3f4a2ab4)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>