docs: Update storage_type docs entry to mention I/O Optimized storage…

… configuration (terraform-aws-modules#393)

README.md

@@ -363,7 +363,7 @@ No modules.
 | <a name="input_snapshot_identifier"></a> [snapshot\_identifier](#input\_snapshot\_identifier) | Specifies whether or not to create this cluster from a snapshot. You can use either the name or ARN when specifying a DB cluster snapshot, or the ARN when specifying a DB snapshot | `string` | `null` | no |
 | <a name="input_source_region"></a> [source\_region](#input\_source\_region) | The source region for an encrypted replica DB cluster | `string` | `null` | no |
 | <a name="input_storage_encrypted"></a> [storage\_encrypted](#input\_storage\_encrypted) | Specifies whether the DB cluster is encrypted. The default is `true` | `bool` | `true` | no |
-| <a name="input_storage_type"></a> [storage\_type](#input\_storage\_type) | Specifies the storage type to be associated with the DB cluster. (This setting is required to create a Multi-AZ DB cluster). Valid values: `io1`, Default: `io1` | `string` | `null` | no |
+| <a name="input_storage_type"></a> [storage\_type](#input\_storage\_type) | Determines the storage type for the DB cluster. Optional for Single-AZ, required for Multi-AZ DB clusters. Valid values for Single-AZ: `aurora`, `""` (default, both refer to Aurora Standard), `aurora-iopt1` (Aurora I/O Optimized). Valid values for Multi-AZ: `io1` (default). | `string` | `null` | no |
 | <a name="input_subnets"></a> [subnets](#input\_subnets) | List of subnet IDs used by database subnet group created | `list(string)` | `[]` | no |
 | <a name="input_tags"></a> [tags](#input\_tags) | A map of tags to add to all resources | `map(string)` | `{}` | no |
 | <a name="input_vpc_id"></a> [vpc\_id](#input\_vpc\_id) | ID of the VPC where to create security group | `string` | `""` | no |

variables.tf

@@ -295,7 +295,7 @@ variable "storage_encrypted" {
 }
 
 variable "storage_type" {
-  description = "Specifies the storage type to be associated with the DB cluster. (This setting is required to create a Multi-AZ DB cluster). Valid values: `io1`, Default: `io1`"
+  description = "Determines the storage type for the DB cluster. Optional for Single-AZ, required for Multi-AZ DB clusters. Valid values for Single-AZ: `aurora`, `\"\"` (default, both refer to Aurora Standard), `aurora-iopt1` (Aurora I/O Optimized). Valid values for Multi-AZ: `io1` (default)."
   type        = string
   default     = null
 }