[llvm] [MISched][NFC] Add documentation comment in pickNode for ReadyQueue maintenence (PR #92976)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

I didn't take a deep look at this part, but I think it makes sense.

https://github.com/llvm/llvm-project/pull/92976