Suggest `gn help` for internally set arguments.

tools/gn/command_args.cc

Index: tools/gn/command_args.cc
diff --git a/tools/gn/command_args.cc b/tools/gn/command_args.cc
index 12c20d75027952c3b06ca9ea0e99ae29a4ae9416..3b3dde627a006924b02ec0529d7474e8d4ee22a2 100644
--- a/tools/gn/command_args.cc
+++ b/tools/gn/command_args.cc
@@ -111,7 +111,8 @@ void PrintArgHelp(const base::StringPiece& name, const Value& value) {
     GetContextForValue(value, &location, &comment);
     OutputString("    " + location + "\n" + comment);
   } else {
-    OutputString("    (Internally set)\n");
+    OutputString("    (Internally set; try `gn help " + name.as_string() +
+                 "`.)\n");
   }
 }