spec: pointer to array can be sliced

This has always been true, but we lost it from the spec
somewhere along the way, probably when we disallowed
the general 'pointer to anything sliceable' slice case.

R=gri
CC=golang-dev
https://golang.org/cl/5437121

doc/go_spec.html

@@ -2440,7 +2440,7 @@ Assigning to an element of a <code>nil</code> map causes a
 <h3 id="Slices">Slices</h3>
 
 <p>
-For a string, array, or slice <code>a</code>, the primary expression
+For a string, array, pointer to array, or slice <code>a</code>, the primary expression
 </p>
 
 <pre>