You've hit on an important point here. As soon as Apple documents one of their CPU extensions, they've put themselves in the position of having to support it long-term. Leaving it undocumented gives them the freedom to make breaking changes from one CPU generation to the next -- hopefully with the end goal of deciding on an implementation and documenting the end result.
1: https://www.anandtech.com/show/14859/apple-announces-new-iph...