doc: clarify non-nil zero length slice to array pointer conversion

There is an example for nil slice already, so adding example for non-nil
zero length slice, too, clarifying to the reader that the result is also
non-nil and different from nil slice case.

Updates #395

Change-Id: I019db1b1a1c0c621161ecaaacab5a4d888764b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/336890
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Jul 24, 2021",
+	"Subtitle": "Version of Jul 26, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -4335,6 +4335,9 @@ <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array
 var t []string
 t0 := (*[0]string)(t)    // t0 == nil
 t1 := (*[1]string)(t)    // panics: len([1]string) > len(t)
+
+u := make([]byte, 0)
+u0 = (*[0]byte)(u)       // u0 != nil
 </pre>
 
 <h3 id="Constant_expressions">Constant expressions</h3>