spec: reject top-level init names

Fixes #4586.

R=golang-dev, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/7307082

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 9, 2013",
+	"Subtitle": "Version of February 11, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -5472,8 +5472,10 @@ func init()
 </pre>
 <p>
 defined in its source.
-A package may contain multiple
-<code>init</code> functions, even
+A package-scope or file-scope identifier
+with name <code>init</code> may only be
+declared to be a function with this signature.
+Multiple such functions may be defined, even
 within a single source file; they execute
 in unspecified order.
 </p>