See title.

For reference:

`GenericAlias` was added by Guido van Rossum <guido@python.org> in commit 48b069a003ba6c684a9ba78493fbbec5e89f10b8 "bpo-39481: Implementation for [PEP 585](https://www.python.org/dev/peps/pep-0585/) (#18239)"

`Union` was added by Maggi Moss <MaggieMoss@users.noreply.github.com> in commit 1b4552c5e8e925f24c15f707050f22c977171125 "bpo-41428: Implementation for [PEP 604](https://www.python.org/dev/peps/pep-0604/) (GH-21515)"

I can do a PR which would need backporting (is that the right word?) to 3.9 at least for `GenericAlias`.

Hi, Patrick! Yes! It would be great if you could open a PR to document those new types.

The documentation of types.Union should go to master only, and the documentation of types.GenericAlias to both master and 3.9, so two PRs should me opened to address this issues separately.

The backporting (yes! that's the word) to 3.9 will be (fingers-crossed) automatic thanks to Mariatta's https://github.com/python/miss-islington

Hi Patrick, I've completed the docs for PEP 604, and would like to work on the docs for PEP 585. May I know if you're still working on this?

Sorry, I'd completely forgotten about doing a PR for this. Go ahead!

New changeset 4173320920706b49a004bdddd8d7108e8984e3fc by kj in branch 'master':
bpo-41805: Documentation for PEP 585 (GH-22615)
https://github.com/python/cpython/commit/4173320920706b49a004bdddd8d7108e8984e3fc

New changeset 577d7c4e628260eb7926d043ca9c355ece583eb7 by kj in branch '3.9':
[3.9] bpo-41805: Documentation for PEP 585 (GH-22615) (GH-23016)
https://github.com/python/cpython/commit/577d7c4e628260eb7926d043ca9c355ece583eb7

Thanks Ken Ji! Are you planning more doc patches?

@guido, you're welcome! I'm thinking of updating the temporary hyperlinks for GenericAlias/PEP 585 in Union, subscriptions and typing, I'll submit 2 separate PRs since subscription and typing require backporting. 

If I think of anything major, I'll open a separate bpo to keep track of it there. Thanks.

Cool. Please add me to the nosy list of any issues you open.

Also, if you're interesting helping out with the match statement, once the SC approves it, we'll need to add docs for that. See issue42128 for a possible plan.