GH-3954: Add docs warning about @PostConstruct & NewTopic beans

Fixes: github.com//issues/3954

* Docs: Improve messaging component guidance based on review feedback

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

Author: Chaedie

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/sending-messages.adoc

@@ -72,6 +72,18 @@ If the topic is configured to use `LOG_APPEND_TIME`, the user-specified timestam
 The `metrics` and `partitionsFor` methods delegate to the same methods on the underlying javadoc:org.apache.kafka.clients.producer.Producer[].
 The `execute` method provides direct access to the underlying javadoc:org.apache.kafka.clients.producer.Producer[].
 
+[NOTE]
+====
+When sending messages from Spring components, avoid using `@PostConstruct` methods if relying on automatic topic creation via `NewTopic` beans.
+`@PostConstruct` runs before the application context is fully ready, which may cause `UnknownTopicOrPartitionException` on clean brokers.
+
+Instead, consider these alternatives:
+
+- Use an `ApplicationListener<ContextRefreshedEvent>` to ensure the context is fully refreshed before sending.
+- Implement `SmartLifecycle` to start after the `KafkaAdmin` bean has completed its initialization.
+- Pre-create topics externally.
+====
+
 To use the template, you can configure a producer factory and provide it in the template's constructor.
 The following example shows how to do so: