r/aws_docdb_cluster(doc): clarify availability_zones behavior (#44265)

Clarifies that specifying less than 3 availability zones may result in persistent differences without an `ignore_changes` lifecycle argument. From the DocumentDB documentation.

> When you create a cluster using the Amazon DocumentDB console, and you choose to create a replica in a different Availability Zone, Amazon DocumentDB creates two instances. It creates the primary instance in one Availability Zone and the replica instance in a different Availability Zone. **The cluster volume is always replicated across three Availability Zones.**

- https://docs.aws.amazon.com/documentdb/latest/developerguide/regions-and-azs.html

Author: jar-b

website/docs/r/docdb_cluster.html.markdown

@@ -46,8 +46,9 @@ This resource supports the following arguments:
 * `apply_immediately` - (Optional) Specifies whether any cluster modifications
      are applied immediately, or during the next maintenance window. Default is
      `false`.
-* `availability_zones` - (Optional) A list of EC2 Availability Zones that
-  instances in the DB cluster can be created in.
+* `availability_zones` - (Optional) A list of EC2 Availability Zones that instances in the DB cluster can be created in.
+  DocumentDB automatically assigns 3 AZs if less than 3 AZs are configured, which will show as a difference requiring resource recreation next Terraform apply.
+  We recommend specifying 3 AZs or using [the `lifecycle` configuration block `ignore_changes` argument](https://www.terraform.io/docs/configuration/meta-arguments/lifecycle.html#ignore_changes) if necessary.
 * `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.