(route53-targets): `@aws-cdk/aws-route53-targets:userPoolDomainNameMethodWithoutCustomResource` feature flag migration is unclear

### Describe the issue

The current documentation for the `@aws-cdk/aws-route53-targets:userPoolDomainNameMethodWithoutCustomResource` feature flag doesn't provide any information about the migration path from the old default to the new.

From what I can tell, the old Custom Resource just looked up the domain name, which is now exposed through a direct CloudFormation export: https://github.com/aws/aws-cdk/blob/db62c5fd7b910b7291a13634b5968f20f89dc271/packages/aws-cdk-lib/aws-cognito/lib/user-pool-domain.ts#L162-L193

And the feature flag just changes this reference: https://github.com/aws/aws-cdk/blob/db62c5fd7b910b7291a13634b5968f20f89dc271/packages/aws-cdk-lib/aws-route53-targets/lib/userpool-domain.ts#L14-L23

So I'm a bit confused why this is described as a "breaking change" in the docs. I made this update to a few projects of mine and the update of the Route53 record was transparent and didn't cause any downtime or problem.

Could we clarify the documentation to make this sound less scary?

### Links

https://github.com/aws/aws-cdk/blob/db62c5fd7b910b7291a13634b5968f20f89dc271/packages/aws-cdk-lib/cx-api/FEATURE_FLAGS.md#L1747-L1762

Start with the feature-flag entry in packages/aws-cdk-lib/cx-api/FEATURE_FLAGS.md and compare the referenced user-pool-domain.ts and userpool-domain.ts implementations. Clarify the migration path from the old default and explain why the change is considered breaking, while accurately reflecting the reported transparent Route 53 update.

Written by the indexing model from the issue text.