Add real comments to yields in qthreads sync var lock and unlock

These were added to resolve deadlock that cropped up when we switched to the
nemesis scheduler. I never commented them properly because we weren't sure it
was the final solution or if the values were good. Things seem to be in good
shape some I'm adding extensive comments.
Might note that if the task that will ultimately grant us access is on the same worker then we *have* to yield or we'll deadlock, while if that other task is on another worker then spinning might let us proceed more quickly once they do release it (and us). That tension between deadlock avoidance and latency minimization is what pushes us to even consider spinning in addition to yielding, and to worry about how much of it we should do.
Also, are we sure that native qthreads syncs would avoid this, or do we just think they might? I thought it was closer to the latter, but the statement as written seems quite confident they would do so.