notes: fence: rename variables to match spec better

Originally, I purposely renamed these since I needed to introduce more
variables than were in the spec. But it turns out I can make this match
the spec pretty well.

Now: A, B, X, and Y fulfill the same roles as in the original C++
specification statements. I only introduce my new variable as W, when
needed. This also makes sense because W is a Write.

Also, I delete an extraneous paragraph that duplicated a piece of the
spec.