Add a line to pub usage output explaining how to see global options.

sdk/lib/_internal/pub/test/global/activate/unexpected_arguments_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('fails if there are extra arguments', () {
     schedulePub(args: ["global", "activate", "foo", "1.0.0", "bar", "baz"],
         error: """
             Unexpected arguments "bar" and "baz".
 
             Usage: pub global activate <package> [version]
             -h, --help    Print usage information for this command.
+
+            Run "pub help" to see global options.
             """,
         exitCode: exit_codes.USAGE);
   });
 }