spec: clarify wording on passing slice arguments to variadic functions

Per discussion on #30769.

Fixes #30769.

Change-Id: I620dbac936de1a0b5deec03926dd11d690a918e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167380


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>