Enhance GN documentation.

This patch updates various GN documentation.

BUG=

Review URL: https://codereview.chromium.org/1367923006

Cr-Original-Commit-Position: refs/heads/master@{#350980}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1c3192f7e36fedcaaa02f262df58e246a3114972