Merge pull request #21675 from yu-croco/update_rds_cluster_doc

chore: update doc of `aws_rds_cluster`, `backtrack_window`  description

website/docs/r/rds_cluster.html.markdown

@@ -108,7 +108,7 @@ The following arguments are supported:
 * `allow_major_version_upgrade` - (Optional) Enable to allow major engine version upgrades when changing engine versions. Defaults to `false`.
 * `apply_immediately` - (Optional) Specifies whether any cluster modifications are applied immediately, or during the next maintenance window. Default is `false`. See [Amazon RDS Documentation for more information.](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Overview.DBInstance.Modifying.html)
 * `availability_zones` - (Optional) A list of EC2 Availability Zones for the DB cluster storage where DB cluster instances can be created. RDS automatically assigns 3 AZs if less than 3 AZs are configured, which will show as a difference requiring resource recreation next Terraform apply. It is recommended to specify 3 AZs or use [the `lifecycle` configuration block `ignore_changes` argument](https://www.terraform.io/docs/configuration/meta-arguments/lifecycle.html#ignore_changes) if necessary.
-* `backtrack_window` - (Optional) The target backtrack window, in seconds. Only available for `aurora` engine currently. To disable backtracking, set this value to `0`. Defaults to `0`. Must be between `0` and `259200` (72 hours)
+* `backtrack_window` - (Optional) The target backtrack window, in seconds. Only available for `aurora` and `aurora-mysql` engines currently. To disable backtracking, set this value to `0`. Defaults to `0`. Must be between `0` and `259200` (72 hours)
 * `backup_retention_period` - (Optional) The days to retain backups for. Default `1`
 * `cluster_identifier_prefix` - (Optional, Forces new resource) Creates a unique cluster identifier beginning with the specified prefix. Conflicts with `cluster_identifier`.
 * `cluster_identifier` - (Optional, Forces new resources) The cluster identifier. If omitted, Terraform will assign a random, unique identifier.