Document support of Vavr's `Try` to potentially roll back declarative transactions

#20361 introduced support for Vavr's `Try` monad to trigger transaction rollbacks when an `@Transactional`-annotated method returns a `Failure`.

I would appreciate it if that feature was described in the reference documentation, likely [this section](https://docs.spring.io/spring-framework/docs/current/reference/html/data-access.html#transaction-declarative-rolling-back).