doc: minor clarification regarding the sharing of underlying arrays

The last sentence in the section on slice expressions could be read
as if it might apply to strings. Changed the sentence a bit to
emphasize its applicability to slices only. See also the issue for
more background.

Fixes #19220.

Change-Id: I8551f34230e4ed93f951e7b871cc81f54a5874a9
Reviewed-on: https://go-review.googlesource.com/59890Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of August 25, 2017",
+	"Subtitle": "Version of August 30, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -3147,8 +3147,8 @@ and the result of the slice operation is a slice with the same element type as t
 
 <p>
 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
-is a <code>nil</code> slice. Otherwise, the result shares its underlying array with the
-operand.
+is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
+array with the operand.
 </p>
 
 <h4>Full slice expressions</h4>