provider/aws: `directory_service_directory` documentation
provider/aws: `directory_service_directory` documentation

Fixes #4648

The AWS Console shows that 2 subnets are required - not a minimum

terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws