[doc] Reword sentinel object summary in dataclasses (GH-27792)

This sentinel value (`MISSING`) is also used as default value for the `kw_only` parameter introduced in Python 3.10. It's cleaner to simply omit the usage here.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
(cherry picked from commit 767a17f35a581da664ac8cf5d67281da9485eebf)

Co-authored-by: Ville Korhonen <ville@xd.fi>