[gndoc] Update GN build arguments documentation

Roller-URL: https://cr-buildbucket.appspot.com/build/8680323932029169137
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia50101b333bdcad73ef0c2d660cad8290907768e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1632994
Commit-Queue: docs-roller@fuchsia-infra.iam.gserviceaccount.com <docs-roller@fuchsia-infra.iam.gserviceaccount.com>
1 file changed