[3.13] gh-133986: Document string split algorithm when sep is None and maxsplit is 0 (GH-133987) (#133992)

gh-133986: Document string split algorithm when sep is None and maxsplit is 0 (GH-133987)
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(cherry picked from commit 3e23047363f384b7254b7af51afe4e353be94167)

Co-authored-by: Joey Smith <joeysmith@gmail.com>
Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>
Co-authored-by: Semyon Moroz <donbarbos@proton.me>