spec: values of underlying type uintptr can be converted to unsafe.Pointer

Not a language change, just stating the facts.

Fixes #1793.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5488071

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of December 12, 2011 -->
+<!-- subtitle Version of December 13, 2011 -->
 
 <!--
 TODO
@@ -5260,7 +5260,7 @@ func Unreflect(typ runtime.Type, addr uintptr) interface{}
 </pre>
 
 <p>
-Any pointer or value of type <code>uintptr</code> can be converted into
+Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted into
 a <code>Pointer</code> and vice versa.
 </p>
 <p>