aws_kms_key: update documentation

website/docs/r/kms_key.html.markdown

@@ -48,6 +48,7 @@ If the KMS key is a multi-Region primary key with replicas, the waiting period b
 * `enable_key_rotation` - (Optional) Specifies whether [key rotation](http://docs.aws.amazon.com/kms/latest/developerguide/rotate-keys.html) is enabled. Defaults to `false`.
 * `multi_region` - (Optional) Indicates whether the KMS key is a multi-Region (`true`) or regional (`false`) key. Defaults to `false`.
 * `tags` - (Optional) A map of tags to assign to the object. 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.
+* `xks_key_id` - (Optional) Identifies the external key that serves as key material for the KMS key in an external key store.
 
 ## Attribute Reference