GH-3001: add docs

spring-kafka-docs/src/main/antora/modules/ROOT/pages/whats-new.adoc

@@ -40,4 +40,32 @@ See xref:retrytopic/features.adoc#exc-based-custom-dlt-routing[Routing of messag
 === After Rollback Processing
 
 A new `AfterRollbackProcessor` API `processBatch` is provided.
-See xref:kafka/annotation-error-handling.adoc#after-rollback[After-rollback Processor] for more information.
+See xref:kafka/annotation-error-handling.adoc#after-rollback[After-rollback Processor] for more information.
+
+[[x32-default-clientid-prefix]]
+=== Spring Boot application name as default client ID prefix
+
+For Spring Boot applications which define an application name using the `spring.application.name` property, this name is now used
+as a default prefix for auto-generated client IDs for these client types:
+
+- consumer clients which don't use a consumer group
+- producer clients
+- admin clients
+
+.Example client ids resulting for a Spring Boot application with `spring.application.name=myapp`
+[%autowidth]
+|===
+|Client Type |Before |After
+
+|consumer without consumer group
+|consumer-null-1
+|myapp-consumer-1
+
+|producer
+|producer-1
+|myapp-producer-1
+
+|admin
+|adminclient-1
+|myapp-admin-1
+|===