[plt-scheme] Patches for new documentation

Thanks very much!

Since I'm now working in a v4.0 branch, I've applied your patches
there.

I turned most of your comments into "FIXME"s in the doc source. One
note, though:

At Fri, 19 Oct 2007 19:38:01 -0700, Eric Hanchrow wrote:
> +;; Anyway, "piece" is a procedure whereas "substring" is a primitive;
> +;; you might want to expound a bit about that, since it's puzzling to
> +;; the uninitiated.

I think the v4.0 printer should always say "procedure" and never
"primitive", so there won't be a distinction to make.


Matthew