Learned of this in https://news.ycombinator.com/item?id=37186869 it was posted 2 years ago to no discussion https://news.ycombinator.com/item?id=30291459 seems far more interesting than that.

https://github.com/facebook/hhvm/blob/master/hphp/hack/doc/H... appears to be where it was proposed, and has more background than the current docs.