spec: describe an edge case for slice expression of nil slice

spec: describe an edge case for slice expression of nil slice

Change-Id: I8c0e2b37e7e8cb4db6ad0b456fde7eb908ffbd04
Reviewed-on: https://go-review.googlesource.com/c/go/+/430836
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>