let-values, and-let* no variables

[Kevin Ryde]

I'm contemplating the way "begin" is used for the body of let-values
and and-let* with no bindings.  It has the unhappy effect of allowing
what is visually an internal define to go out into the containing
environment, eg.

        (use-modules (srfi srfi-11))
        (let-values ()
          (define x 123)
          456)
        (display x)
        ;; prints 123

The reference implementation of srfi-11 seems pretty clear that
"begin" is intended for let-values, and the formal semantics part of
srfi-2 suggests the same for and-let*.  Dunno if that's only an
oversight regarding begin versus internal defines though.

I guess nobody would write an empty and-let* or let-values
deliberately, but it could arise out of a macro.  I'm thinking to put
some words of caution in the manual, to make it clear the body isn't
the same as a plain "let" or "let*".