doc: add missing builtin functions not permitted in statement context

doc: add missing builtin functions not permitted in statement context

The typechecker already enforces this semantic, but the spec is not
updated when unsafe.{SliceData,String,StringData} were added.

Change-Id: I4ee8c564d5681b2a5fd31ff424a31bdf065d9f3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/467756
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>