spec: clarify value passed for final parameter of variadic functions
spec: clarify value passed for final parameter of variadic functions

NOT A LANGUAGE CHANGE.

Fixes issue 7073.

LGTM=r
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/68840045

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang