spec: better wording re: receiving from closed channels

As suggested by ej@evanjones.ca.

Fixes #5604.

R=r
CC=golang-dev
https://golang.org/cl/9899043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 20, 2013",
+	"Subtitle": "Version of May 31, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -3301,9 +3301,8 @@ from the channel <code>ch</code>. The channel direction must permit receive oper
 and the type of the receive operation is the element type of the channel.
 The expression blocks until a value is available.
 Receiving from a <code>nil</code> channel blocks forever.
-Receiving from a <a href="#Close">closed</a> channel always succeeds,
-immediately returning the element type's <a href="#The_zero_value">zero
-value</a>.
+A receive operation on a <a href="#Close">closed</a> channel can always proceed
+immediately, yielding the element type's <a href="#The_zero_value">zero value</a>.
 </p>
 
 <pre>