spec: clarify nil case in type switches

The old wording seemed to imply that nil is a kind of type.
Slightly reworded for clarity.

Fixes #21580.

Change-Id: I29898bf0125a10cb8dbb5c7e63ec5399ebc590ca
Reviewed-on: https://go-review.googlesource.com/58490
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of August 14, 2017",
+	"Subtitle": "Version of August 25, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -4820,8 +4820,9 @@ <h4 id="Type_switches">Type switches</h4>
 </p>
 
 <p>
-The type in a case may be <a href="#Predeclared_identifiers"><code>nil</code></a>;
-that case is used when the expression in the TypeSwitchGuard
+Instead of a type, a case may use the predeclared identifier
+<a href="#Predeclared_identifiers"><code>nil</code></a>;
+that case is selected when the expression in the TypeSwitchGuard
 is a <code>nil</code> interface value.
 There may be at most one <code>nil</code> case.
 </p>