Adding documentation for connecting to multiple Kafka clusters with separate JAAS configuraiton from within a single application. Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/944 Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/874 Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/924