protogen: clarify that GoIdent.GoName is a single idenfitier

While we're at it, also add the license header for the file.

Change-Id: If33f1764111aff96efcdb5a4e88c2a70ae28c09c
Reviewed-on: https://go-review.googlesource.com/c/150060
Reviewed-by: Damien Neil <dneil@google.com>

protogen/names.go

@@ -1,3 +1,7 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package protogen
 
 import (
@@ -12,6 +16,7 @@ import (
 )
 
 // A GoIdent is a Go identifier, consisting of a name and import path.
+// The name is a single identifier and may not be a dot-qualified selector.
 type GoIdent struct {
 	GoName       string
 	GoImportPath GoImportPath