Make Outputable instance for HsDocString useful

Seeing "<document comment>" when trying to debug things in Haddock is
utterly useless and because this instance exists, we can't even make our
own. No instance at all would be better than what it was!

Admittedly, this doesn't produce the nicest output. Perhaps wrapping the
comments in {- -} would be in order but I think it's fine until someone
complains.