better document razoring
The comment about returning v + margin is incorrect, and induces people into erroneous reasoning.
Going back to known theory about alpha/beta, there are two ways of pruning:
* fail hard: return alpha
* fail soft: return v
This business of returning v + margin (or v - margin as was discussed on the forum), I have no idea where it comes from!
It no basis in theory. That doesn't mean it cannot work, of course: only testing can answer that question.

Also, use ralpha instead of rbeta:
* rbeta is confusing people. It took THREE attempts to code razoring at PV nodes correctly in a recent test, because of the rbeta trick. Unnecessary tricks should be avoided.
* The more correct and self-documenting way of doing this, is to say that we use a zero window around alpha-margin, not beta-margin. The fact that, because we only do it at PV nodes, alpha happens to be beta-1 and that the current stuff with rbeta works, may be correct, but is confusing.

No functional change.