Clarify the instance type to use for Aurora Serverless v2

When creating an Aurora Serverless v2 cluster in Terraform, users
must add an instance to the cluster. The documentation points to the
AWS documentation page on the currently available instance classes,
but this documentation does not include `db.serverless`.

The Aurora Serverless v2 documentation itself also has scant mention
of it.

Including the information here as well ought to help users.

website/docs/r/rds_cluster_instance.html.markdown

@@ -67,7 +67,7 @@ This resource supports the following arguments:
   Valid Values: `aurora-mysql`, `aurora-postgresql`, `mysql`, `postgres`.(Note that `mysql` and `postgres` are Multi-AZ RDS clusters).
 * `identifier_prefix` - (Optional, Forces new resource) Creates a unique identifier beginning with the specified prefix. Conflicts with `identifier`.
 * `identifier` - (Optional, Forces new resource) Identifier for the RDS instance, if omitted, Terraform will assign a random, unique identifier.
-* `instance_class` - (Required) Instance class to use. For details on CPU and memory, see [Scaling Aurora DB Instances][4]. Aurora uses `db.*` instance classes/types. Please see [AWS Documentation][7] for currently available instance classes and complete details.
+* `instance_class` - (Required) Instance class to use. For details on CPU and memory, see [Scaling Aurora DB Instances][4]. Aurora uses `db.*` instance classes/types. Please see [AWS Documentation][7] for currently available instance classes and complete details. For Aurora Serverless v2 use `db.serverless`.
 * `monitoring_interval` - (Optional) Interval, in seconds, between points when Enhanced Monitoring metrics are collected for the DB instance. To disable collecting Enhanced Monitoring metrics, specify 0. The default is 0. Valid Values: 0, 1, 5, 10, 15, 30, 60.
 * `monitoring_role_arn` - (Optional) ARN for the IAM role that permits RDS to send enhanced monitoring metrics to CloudWatch Logs. You can find more information on the [AWS Documentation](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Monitoring.html) what IAM permissions are needed to allow Enhanced Monitoring for RDS Instances.
 * `performance_insights_enabled` - (Optional) Specifies whether Performance Insights is enabled or not.