doco scm_values

[Kevin Ryde]

A bit of a tweak and addition for "values",

        * scheme-control.texi (Multiple Values): In values, show args as "arg1
        ... argN".  In scm_values, note args is a list and that the returned
        object shares structure with it.


 - Scheme Procedure: values arg1 ... argN
 - C Function: scm_values (args)
     Delivers all of its arguments to its continuation.  Except for
     continuations created by the `call-with-values' procedure, all
     continuations take exactly one value.  The effect of passing no
     value or more than one value to continuations that were not
     created by `call-with-values' is unspecified.

     For `scm_values', ARGS is a list and the return is a
     multiple-values object which the caller can return.  In the current
     implementation, that object shares structure with ARGS, so ARGS
     should not be modified later.