[3.14] gh-134976: document the exception type that can be raised by `s[i]` (GH-134977) (#135258)

gh-134976: document the exception type that can be raised by `s[i]` (GH-134977)
(cherry picked from commit 158e5162bfaa8a49178ce2c3f2455c3e03b60157)

Co-authored-by: Yongzi Li <204532581+Yzi-Li@users.noreply.github.com>