ALS implicit: added missing parameter alpha in doc string

So, while we're here, maybe clean a few more things up. "immplicit" is misspelled, and since this is documentation for `trainImplicit`, there is no `implicitPrefs` parameter anyway. It's, erm, _implicitly_ true. But these were just true in the text you copied, so can perhaps be fixed elsewhere.

It's really 1 + alpha*R that is the confidence value. alpha sort of controls how difference confidence is in an observed and unobserved interaction. Maybe it's nice to bother to explain a tiny bit more about what this is, in the class scaladoc, and refer to it. While we're here.