[plt-scheme] documentation feature requests

Robby Findler wrote:
> I've seen that Jon Rafkind has been adding such things to lots of
> places in the docs and I've put them in the new image documentation
> library (http://pre.plt-scheme.org/docs/html/teachpack/2htdpimage.html),
> so I think it is safe to say we recognize the need and are working on
> it.
>
>   

I just added examples for all the functions in section 3.9 except for 
the immutable cyclic data stuff at the bottom.

The spacing for the caar/cddr/etc examples is broken, if anyone wants to 
help me out.