Fix sns/publish docs and contract. Fixes #34.

- The Scribble was wrong for the `subject` and `json?` parameters.

- The contract for `subject` was too permissive; `#f` shouldn't be
  allowed.

- Note that it's OK for the return contract to be `any` even though the
  docs say `void?`. Using `any` instead of `void?` is faster and AFAICT
  it's a common practice to use `any` but doc it as `void?`.