aws_rds_global_cluster: add clarity on updating global cluster versio…

…ns (#41127)

website/docs/r/rds_global_cluster.html.markdown

@@ -220,7 +220,7 @@ This resource supports the following arguments:
 * `engine_lifecycle_support` - (Optional) The life cycle type for this DB instance. This setting applies only to Aurora PostgreSQL-based global databases. Valid values are `open-source-rds-extended-support`, `open-source-rds-extended-support-disabled`. Default value is `open-source-rds-extended-support`. [Using Amazon RDS Extended Support]: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/extended-support.html
 * `engine_version` - (Optional) Engine version of the Aurora global database. The `engine`, `engine_version`, and `instance_class` (on the `aws_rds_cluster_instance`) must together support global databases. See [Using Amazon Aurora global databases](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-global-database.html) for more information. By upgrading the engine version, Terraform will upgrade cluster members. **NOTE:** To avoid an `inconsistent final plan` error while upgrading, use the `lifecycle` `ignore_changes` for `engine_version` meta argument on the associated `aws_rds_cluster` resource as shown above in [Upgrading Engine Versions](#upgrading-engine-versions) example.
 * `force_destroy` - (Optional) Enable to remove DB Cluster members from Global Cluster on destroy. Required with `source_db_cluster_identifier`.
-* `source_db_cluster_identifier` - (Optional) Amazon Resource Name (ARN) to use as the primary DB Cluster of the Global Cluster on creation. Terraform cannot perform drift detection of this value.
+* `source_db_cluster_identifier` - (Optional) Amazon Resource Name (ARN) to use as the primary DB Cluster of the Global Cluster on creation. Terraform cannot perform drift detection of this value. **NOTE:** After initial creation, this argument can be removed and replaced with `engine` and `engine_version`. This allows upgrading the engine version of the Global Cluster.
 * `storage_encrypted` - (Optional, Forces new resources) Specifies whether the DB cluster is encrypted. The default is `false` unless `source_db_cluster_identifier` is specified and encrypted. Terraform will only perform drift detection if a configuration value is provided.
 * `tags` - (Optional) A map of tags to assign to the DB cluster. If configured with a provider [`default_tags` configuration block](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#default_tags-configuration-block) present, tags with matching keys will overwrite those defined at the provider-level.