Suggest `gn help` for internally set arguments.

tools/gn/command_args.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include <map>
@@ skipping 93 matching lines @@
 
 void PrintArgHelp(const base::StringPiece& name, const Value& value) {
   OutputString(name.as_string(), DECORATION_YELLOW);
   OutputString("  Default = " + value.ToString(true) + "\n");
 
   if (value.origin()) {
     std::string location, comment;
     GetContextForValue(value, &location, &comment);
     OutputString("    " + location + "\n" + comment);
   } else {
-    OutputString("    (Internally set)\n");
+    OutputString("    (Internally set; try `gn help " + name.as_string() +
+                 "`.)\n");
   }
 }
 
 int ListArgs(const std::string& build_dir) {
   Setup* setup = new Setup;
   setup->build_settings().set_check_for_bad_items(false);
   if (!setup->DoSetup(build_dir, false) || !setup->Run())
     return 1;
 
   Scope::KeyValueMap build_args;
@@ skipping 218 matching lines @@
         "Or see \"gn help args\" for more variants.").PrintToStdout();
     return 1;
   }
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchList))
     return ListArgs(args[0]);
   return EditArgsFile(args[0]);
 }
 
 }  // namespace commands