spec: more precise prose for special case of append
As written, the special case for append may be interpreted
such that any first argument that is assignable to []byte
is permissible, including nil. This change makes makes it
clear that a slice argument is required.
Not a language change. The compiler always expected a slice
argument.
Fixes #76226.
Change-Id: I73a1725d10096690335d6edf5793beb5de73578f
Reviewed-on: https://go-review.googlesource.com/c/go/+/718880 Reviewed-by: Junyang Shao <shaojunyang@google.com> Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>