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.
For qthreads sync vars a lock will put the calling task to sleep if the lock isn't free and once the lock is released, one task will be woken up. So yes, I believe this is true, though this doesn't get rid of the need for the yield in unlock (and maybe a "real" solution there requires some changes to qthreads as it seems like a problem they would want addressed as well