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

I had some trouble understanding why `removeReady` removed nodes from
the Pending queue, since my intuition told me that the Pending queue did
not represent a node that was ready. I took a deeper look and found that
pickOnlyNode and pickNodeFromQueue only picked nodes from the Available
queue too.

I found that need to nodes from the Available and Pending queues that
correspond to the opposite direction that we ended up choosing from
(IsTopNode vs !IsTopNode).

It took me a little longer than I would have liked to understand this
fact, so I figured that I would add a comment in the code that makes it
clear for future readers.