Document the 22.0.0-rc.1 changes #3834

Docs merged to main show on GitHub as the current docs as soon as they land. The docs for 22.0.0-rc.1 describe behavior that ships only in canaries until the 22.0.0-rc.1 tag. Merged one at a time, they would tell readers of the current docs about behavior their installed version does not have yet.

To avoid that, the docs for every 22.0.0-rc.1 change should be collected in one docs PR, which merges just before the tag so the docs on main and the published package change together.

This issue tracks that PR and closes when it merges.