spec: unsafe.Add/Slice are not permitted in statement context

Add unsafe.Add and unsafe.Slice to the list of built-in functions
which are not permitted in statement context. The compiler and
type checker already enforce this restriction, this just fixes
a documentation oversight.

For #19367.
For #40481.

Change-Id: Iabc63a8db048eaf40a5f5b5573fdf00b79d54119
Reviewed-on: https://go-review.googlesource.com/c/go/+/329925


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>