[racket] for/hash and for/list: adding nothing, adding more than one thing

> thoroughly enough and didn't realize it does not work outside its own module. I've since fixed this and added the documentation in markdown format (Scribble does some weird extra ```racket additions with markdown that I had to manually remove, too. Dunno what that's about).

I'm sorry, that's a bug with @examples and @interactions (@racketblock is OK).

Jens pointed it out to me yesterday afternoon. This morning I
submitted a pull request:
<https://github.com/plt/racket/pull/296>