Suggest `gn help` for internally set arguments.

Hopefully this will help point new users to the existing documentation.

Review-Url: https://codereview.chromium.org/2134933002
Cr-Original-Commit-Position: refs/heads/master@{#404714}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 45a841b0259335a05874d2ab3acabcc4e3edfe73

tools/gn/command_args.cc

diff --git a/tools/gn/command_args.cc b/tools/gn/command_args.cc
index 12c20d7..3b3dde6 100644
--- a/tools/gn/command_args.cc
+++ b/tools/gn/command_args.cc
@@ -111,7 +111,8 @@
     GetContextForValue(value, &location, &comment);
     OutputString("    " + location + "\n" + comment);
   } else {
-    OutputString("    (Internally set)\n");
+    OutputString("    (Internally set; try `gn help " + name.as_string() +
+                 "`.)\n");
   }
 }