Show doc (if any) not just bluebox.

Also ,describe now returns plain text rather than an association list,
since none of the other information is really useful. Even the
Definition and Documentation buttons can be done using racket-doc and
racket-visit-definition commands on the symbol.

One drawback is that my conversion from HTML to plain text is not always
pleasing. So next I'm going to try having ,describe return HTML text,
then use libxml-parse-html-region to parse it and shr-insert-document to
display it. I may need to simplify the Scribble HTML, however.