GH-3954: Add warning about @PostConstruct timing with NewTopic beans

Signed-off-by: Chaedie <[email protected]>

Author: Chaedie

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/configuring-topics.adoc

@@ -4,6 +4,20 @@
 If you define a `KafkaAdmin` bean in your application context, it can automatically add topics to the broker.
 To do so, you can add a `NewTopic` `@Bean` for each topic to the application context.
 Version 2.3 introduced a new class `TopicBuilder` to make creation of such beans more convenient.
+
+[NOTE]
+====
+When relying on topic auto-creation by declaring `NewTopic` beans, avoid producing
+messages from `@PostConstruct` methods. `@PostConstruct` runs before
+`KafkaAdmin` applies `NewTopic` declarations. On a clean broker, this may cause
+`UnknownTopicOrPartitionException`.
+
+Instead, consider:
+- Using an `ApplicationListener<ContextRefreshedEvent>` to send after the context is fully refreshed.
+- Implementing `SmartLifecycle` and starting after `KafkaAdmin`.
+- Or pre-creating topics externally.
+====
+
 The following example shows how to do so:
 
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