[3.12] Docs: rework the dbm.dumb introduction (GH-114550) (#114620)

(cherry picked from commit 6c2b419fb91c8d7daa769d39f73768114b5eb45a)

- consistently use correct parameter markup
- consistently use submodule name as database name
- improve accuracy of the dbm.dumb.open() spec
- remove dumbdbm class refs and replace them with generic "database object"
- use parameter list for dbm.dumb.open()
(cherry picked from commit 6c2b419fb91c8d7daa769d39f73768114b5eb45a)

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