add subtext to help users migrate

.chloggen/kafka-receiver-standardize-default-topic.yaml

@@ -15,7 +15,11 @@ issues: [27292]
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-subtext:
+subtext: |
+  If you are using the Kafka receiver in a logs and/or a metrics pipeline
+  and you are not customizing the name of the topic to read from with the `topic` property,
+  the receiver will now read from `otlp_logs` or `otlp_metrics` topic instead of `otlp_spans` topic.
+  To maintain previous behavior, set the `topic` property to `otlp_spans`.
 
 # If your change doesn't affect end users or the exported elements of any package,
 # you should instead start your pull request title with [chore] or use the "Skip Changelog" label.