[gndoc] Update GN build arguments documentation

Roller-URL: https://cr-buildbucket.appspot.com/build/8680505149359100497
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia901ac41ab449a2d421bedfa2f12fd9eddd37b87
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1629950
Commit-Queue: docs-roller@fuchsia-infra.iam.gserviceaccount.com <docs-roller@fuchsia-infra.iam.gserviceaccount.com>
1 file changed