Update gn's depfile help text to include more guidance.

Review-Url: https://codereview.chromium.org/2349743002
Cr-Original-Commit-Position: refs/heads/master@{#419555}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 76097c7f435951726c7eaedc728271c084b5517d
1 file changed
tree: 5d49f3183cddc4d880299a7a48abc6862aa70473
  1. tools/