spec: clarify that any unsafe.Pointer type is okay in conversion

The spec is not clear about whether this is allowed or not,
but both compilers allow it, because the reflect implementation
takes advantage of it. Document current behavior.

Fixes #4679.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7303064

doc/go_spec.html

@@ -5610,9 +5610,18 @@ func Sizeof(variable ArbitraryType) uintptr
 </pre>
 
 <p>
-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.
+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.
 </p>
+
+<pre>
+var f float64
+bits = *(*uint64)(unsafe.Pointer(&amp;f))
+
+type ptr unsafe.Pointer
+bits = *(*uint64)(ptr(&amp;f))
+</pre>
+
 <p>
 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>