For simplicity, I'd document that Mahout works on 0.19 and 0.20, and may work on 0.18. That's more what people need to know, rather than confuse the issue with talk of old/new APIs, since even I am confused about what's going on. The two are blending together, while one is deprecated, and it causes problems.
In the one case here, there are two implementations, covering all bases.
