aws-cdk-lib: NotificationARN must be passed by constructor to actually send notifications

### Describe the issue

Spent quite a bit of time on this, thinking that i was doing something wrong. I thought that we could just push to the notificationARNs array, but turns out the cloudformation output isn't produced in the template. I had to pass the array from the constructor (as props) to make it work. I feel like this should be mentioned in the documentation, and i'm willing to submit a PR myself.

### Links

https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.NestedStack.html

Start with the linked aws-cdk-lib.NestedStack API documentation and review how notificationARNs and constructor props are described. Document that passing NotificationARNs through the constructor is required for the CloudFormation template to include notifications, then verify the published API wording reflects this behavior.

Written by the indexing model from the issue text.