[3.13] gh-121567: Improve `slice` C-API docs by mentioning exceptions (GH-121568) (#121578)

gh-121567: Improve `slice` C-API docs by mentioning exceptions (GH-121568)
(cherry picked from commit 84a5597b08b7d53aced2fbd0048271ce762807a8)

Co-authored-by: sobolevn <mail@sobolevn.me>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>