Docs & reno for multi-IR staged pass manager

[Cryoris]

We need to write module-level docs and a release note for the multi-IR pass manager system (#16276, #16291 and #16296). This can happen post-rc1 and this issue is here so we don't forget about it!

[Cryoris]

Thanks for the interest -- this is just a reminder to us (or to me) to not forget the release notes in the end. You can check out the other open issues if you'd like to contribute!