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.
I used to agree with you that "if the task that will ultimately grant us access is on the same worker then we have to yield or we'll deadlock" but I'm not so sure anymore (though I believe you over me on this, so probably just need some convincing) 

If a task is scheduled to run on the same worker as the task that is going to yield, I don't think theres anyway the task that is waiting can have the lock. In the unlock portion we yield after unlocking so I don't think that could cause it. Oh... is there a way a user could cause this? 

Something like 
syncVar.lock (not a real function but whatever)
chpl_task_yield()
syncvar.unlock

// in some other thread
syncVar.lock