Update the documentation to make feature_name Optional

Author: tabito-hara

website/docs/r/rds_cluster_role_association.html.markdown

@@ -29,7 +29,7 @@ This resource supports the following arguments:
 
 * `region` - (Optional) Region where this resource will be [managed](https://docs.aws.amazon.com/general/latest/gr/rande.html#regional-endpoints). Defaults to the Region set in the [provider configuration](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#aws-configuration-reference).
 * `db_cluster_identifier` - (Required) DB Cluster Identifier to associate with the IAM Role.
-* `feature_name` - (Required) Name of the feature for association. This can be found in the AWS documentation relevant to the integration or a full list is available in the `SupportedFeatureNames` list returned by [AWS CLI rds describe-db-engine-versions](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-engine-versions.html).
+* `feature_name` - (Optional) Name of the feature for association. This can be found in the AWS documentation relevant to the integration or a full list is available in the `SupportedFeatureNames` list returned by [AWS CLI rds describe-db-engine-versions](https://docs.aws.amazon.com/cli/latest/reference/rds/describe-db-engine-versions.html).
 * `role_arn` - (Required) Amazon Resource Name (ARN) of the IAM Role to associate with the DB Cluster.
 
 ## Attribute Reference