Add more documentation for GN header checking

Several people have been confused how the header checker works. This
significantly expands the documentation.

Review URL: https://codereview.chromium.org/1820493002

Cr-Original-Commit-Position: refs/heads/master@{#382106}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b24c0792cfe4babd048c7ba1d8031b4fd497a811
6 files changed
tree: ec7aad32660524542b2dd01cdd1c96b67227d7f3
  1. tools/