[racket] Standard references in the docs

Hi,

I have a quite solid Common Lisp background and I'm currently
making myself familiar with Scheme and Racket. It's a refreshing
experience; where CL is like an overgrown jungle, Racket is like
a well-tended garden. :)

You folks also did a tremendous job on documenting it all. Thanks
for that!

The only thing I really did miss so far in the docs is information
on where certain parts are coming from. It would be quite helpful
to have short annotations like "This is part of R5RS", "This comes
from SRFI-39", "This is a Racket extension", etc.

Would that make sense? Thank you.

  Leslie