Update BigQuery Dataset IAM docs to use references

[slevenick]

Upstreams: hashicorp/terraform-provider-google#9927

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 16 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 1 file changed, 15 insertions(+), 3 deletions(-))