[gndoc] Update GN build arguments documentation

Roller-URL: https://cr-buildbucket.appspot.com/build/8672123563612879361
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If3928fb643f69fe90b2649af88c8f0d70bd37471
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1792838
Commit-Queue: docs-roller@fuchsia-infra.iam.gserviceaccount.com <docs-roller@fuchsia-infra.iam.gserviceaccount.com>
1 file changed