docs(opensearchservice): change default value's description for `mult…

…iAzWithStandbyEnabled` (#29634)

### Reason for this change

Default value of the `multiAzWithStandbyEnabled` parameter will be set to true if the feature flag `multiAzWithStandbyEnabled` is enabled now.

However, the current description for the default value does not yet reflect that.

### Description of changes

Change the description for the default value with the feature flag.

### Description of how you validated changes

Nothing. It is just to change the description.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts

@@ -78,7 +78,8 @@ export interface CapacityConfig {
    * For more information, see [Multi-AZ with
    * Standby](https://docs.aws.amazon.com/opensearch-service/latest/developerguide/managedomains-multiaz.html#managedomains-za-standby)
    *
-   * @default - no multi-az with standby
+   * @default - multi-az with standby if the feature flag `ENABLE_OPENSEARCH_MULTIAZ_WITH_STANDBY`
+   * is true, no multi-az with standby otherwise
    */
   readonly multiAzWithStandbyEnabled?: boolean;
 }