spec: clarify that slice a expression shares underlying array with operand

The spec was not very precise as to what happens with respect to sharing
if a sliced operand is (a pointer to) an array. Added a small clarification
and a supporting example.

Fixes #31689.

Change-Id: Ic49351bec2033abd3f5428154ec3e9a7c2c9eaa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/177139Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 13, 2019",
+	"Subtitle": "Version of May 14, 2019",
 	"Path": "/ref/spec"
 }-->
 
@@ -3262,6 +3262,14 @@ is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its
 array with the operand.
 </p>
 
+<pre>
+var a [10]int
+s1 := a[3:7]   // underlying array of s1 is array a; &s1[2] == &a[5]
+s2 := s1[1:4]  // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
+s2[1] = 42     // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
+</pre>
+
+
 <h4>Full slice expressions</h4>
 
 <p>