[racket] Scribble srcdoc and macros

I've added `form-doc` to `scribble/srcdoc`.

Otherwise, I don't think there's a good way to use `thing-doc` to
document syntactic forms.

At Fri, 15 Aug 2014 13:11:55 -0700, Jack Firth wrote:
> Is there a form similar to proc-doc/names but for providing syntactic forms
> instead? Something where it expands to using @defform instead of @defproc.
> It looks like thing-doc might work, but that's meant for arbitrary values
> and I'm really looking for something specifically meant for exporting
> syntactic forms. If no such thing exists, how would an implementation based
> on thing-doc look exactly?
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users