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
1 file changed
tree: 2155801e4114c61913af09837b1d5b154f3ce75c
  1. tools/