cmd/compile/internal/gc: fix ONAME documentation

Named constants are represented as OLITERAL with n.Sym != nil.

Change-Id: If6bc8c507ef8c3e4e47f586d86fd1d0f20bf8974
Reviewed-on: https://go-review.googlesource.com/c/145198Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -574,7 +574,7 @@ const (
 	OXXX Op = iota
 
 	// names
-	ONAME    // var, const or func name
+	ONAME    // var or func name
 	ONONAME  // unnamed arg or return value: f(int, string) (int, error) { etc }
 	OTYPE    // type name
 	OPACK    // import