[3.10] bpo-46120: State that `|` is preferred over `Union` (GH-30222) (GH-30250)

Co-authored-by: Éric <merwok@netwok.org>
Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
(cherry picked from commit 1b30660c3b40da557050948ac8635f2dc50c4ee2)


Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Automerge-Triggered-By: GH:gpshead