Improve agent docs for gn builds

I got annoyed when it overwrote my GN configs once and noticed a few
missing details (e.g. vias)

Change-Id: I29b60d2b7068b21681ce252346ad6263aadd8180
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1253036
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>