Docs tweak for #1408 / #1409: Kafka PubSub using metadata as headers

Signed-off-by: Phil Kedy <phil.kedy@gmail.com>

daprdocs/content/en/reference/components-reference/supported-pubsub/setup-apache-kafka.md

@@ -136,6 +136,20 @@ curl -X POST http://localhost:3500/v1.0/publish/myKafka/myTopic?metadata.partiti
       }'
 ```
 
+### Message headers
+
+All other metadata key/value pairs (that are not `partitionKey`) are set as headers in the Kafka message. For example, here is an example setting a `correlationId` for the message.
+
+```shell
+curl -X POST http://localhost:3500/v1.0/publish/myKafka/myTopic?metadata.correlationId=myCorrelationID \
+  -H "Content-Type: application/json" \
+  -d '{
+        "data": {
+          "message": "Hi"
+        }
+      }'
+```
+
 ## Create a Kafka instance
 
 {{< tabs "Self-Hosted" "Kubernetes">}}