bpo-41805: Documentation for PEP 585 by Fidget-Spinner · Pull Request #22615 · python/cpython
This PR (if accepted) probably needs a backport to 3.9. It also partly addresses https://bugs.python.org/issue40814.
Also, I'll need to update the links in stdtypes Union Type to point to GenericAlias now. However, I'm not sure if doing that in this PR would make backporting harder/go against the 1 issue per PR recommendation.
Lastly, I have a feeling that we should create a section in stdtypes called "Types for Type Annotations" and place GenericAlias and Union under that since it's becoming a moderate chunk of the documentation for that page. I'll open a bpo if that makes sense.