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.
We don't have the lock until (l == s->lockers_out). All we've done prior to yielding or spinning in the loop body is to put in our request to own it.