spec: clarify unsafe.Pointer conversions

A pointer type of underlying type unsafe.Pointer can be used in
unsafe conversions. Document unfortunate status quo.

Fixes #19306.

Change-Id: I28172508a200561f8df366bbf2c2807ef3b48c97
Reviewed-on: https://go-review.googlesource.com/42132Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 3, 2017",
+	"Subtitle": "Version of May 9, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -6431,7 +6431,7 @@ func Sizeof(variable ArbitraryType) uintptr
 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
 value may not be <a href="#Address_operators">dereferenced</a>.
 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to
-a <code>Pointer</code> type and vice versa.
+a type of underlying type <code>Pointer</code> and vice versa.
 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
 </p>