[3.12] Docs: reword sentences about dbm submodule traits (GH-114609) (#115462)

Don't repeatedly say that keys and values are coerced into bytes.
(cherry picked from commit 81e140d10b77f0a41a5581412e3f3471cc77981f)

Co-authored-by: Erlend E. Aasland <erlend@python.org>