[3.14] gh-134833: improve docs for `del s[i:j]` in `Mutable Sequence Types` (GH-134834) (#136608)

gh-134833: improve docs for `del s[i:j]` in `Mutable Sequence Types` (GH-134834)
(cherry picked from commit 609d5adc7cc241da8fe314a64ddd2c8a883ee8b7)

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