[3.10] bpo-38908: [docs] Add changes to 3.10 whatsnew and fix some minor inaccuracies in news (GH-26096) (GH-26337)

The fix only applies to ``isinstance``. ``issubclass`` isn't affected (because it was always working to begin with). So I also fixed the news to reflect that.
(cherry picked from commit 8450e8a81f6d54f45e1fc5c13a03878c9978750d)


Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>