Currently, I'm thinking of adding man_made=yes, as the wiki says that artificial= has been abandoned? 

And then add a description to the point explaining that it's an artificial decorative waterfall to reduce confusion around it (for other editors and users)? I mean, it does seem like a trolltag, but the lack of a tag also seems misleading too. Since there's no other way to clearly describe this feature, I think this is the best course of action.