clarify rule for channel send

R=rsc
CC=go-dev
http://go/go-review/1018002

doc/go_spec.html

@@ -2528,8 +2528,9 @@ For operations involving constants only, see the section on
 </p>
 
 <p>
-In a channel send, the first operand is always a channel and the
-second is a value of the channel's element type.
+In a channel send, the first operand is always a channel and the second
+must be a value <a href="#Assignment_compatibility">assignment compatible</a>
+with the channel's element type.
 </p>
 
 <p>