add GKE default disk_type notes (#14930)

Author: translucens

mmv1/third_party/terraform/website/docs/r/container_cluster.html.markdown

@@ -890,7 +890,7 @@ The `master_authorized_networks_config.cidr_blocks` block supports:
     Prefer configuring `boot_disk`.
 
 * `disk_type` - (Optional) Type of the disk attached to each node
-    (e.g. 'pd-standard', 'pd-balanced' or 'pd-ssd'). If unspecified, the default disk type is 'pd-balanced' This is being migrated to `boot_disk.disk_type`, and must match if specified in both places. Prefer configuring `boot_disk`.
+    (e.g. 'pd-standard', 'pd-balanced', 'pd-ssd', or 'hyperdisk-balanced'). Defaults to `hyperdisk-balanced` if `hyperdisk-balanced` is supported and `pd-balanced` is not supported for the machine type; otherwise defaults to `pd-balanced`. This is being migrated to `boot_disk.disk_type`, and must match if specified in both places. Prefer configuring `boot_disk`.
 
 * `enable_confidential_storage` - (Optional) Enabling Confidential Storage will create boot disk with confidential mode. It is disabled by default.
 
@@ -1085,7 +1085,7 @@ sole_tenant_config {
     in GB. The smallest allowed disk size is 10GB. Defaults to 100GB. This is being migrated from `node_config.disk_size_gb`, and must match if specified in both places. Prefer using this field.
 
 * `disk_type` - (Optional) Type of the disk attached to each node
-    (e.g. 'pd-standard', 'pd-balanced', 'pd-ssd', 'hyperdisk-balanced'). If unspecified, the default disk type is 'pd-balanced' This is being migrated from `node_config.disk_type`, and must match if specified in both places. Prefer using this field.
+    (e.g. 'pd-standard', 'pd-balanced', 'pd-ssd', or 'hyperdisk-balanced'). Defaults to `hyperdisk-balanced` if `hyperdisk-balanced` is supported and `pd-balanced` is not supported for the machine type; otherwise defaults to `pd-balanced`. This is being migrated from `node_config.disk_type`, and must match if specified in both places. Prefer using this field.
 
 * `provisioned_iops` - (Optional) Configure disk IOPs. This is only valid if the `disk_type` is 'hyperdisk-balanced'. See [performance limit documention](https://cloud.google.com/compute/docs/disks/hyperdisk-perf-limits) for more information about valid values.