[3.12] gh-111622: Fix doc for items views (GH-112051) (#112052)

They are set-like even when some values are not hashable,
but work even better when all are.
(cherry picked from commit e31d65e0b7bb6d6fee4e8df54e10976b4cfab1de)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>