Add a note on replication factor value (#3348)

* Add a note on replication factor value

We've got a bug [1] report for a misconfiguration of
`default.topic.replication.factor`.
This PR adds a note that could alert users on the
value of this configuration.

[1] knative-extensions/eventing-kafka-broker#720

* Update docs/eventing/broker/kafka-broker.md

Co-authored-by: Ashleigh Brennan <40172997+abrennan89@users.noreply.github.com>

Co-authored-by: Ashleigh Brennan <40172997+abrennan89@users.noreply.github.com>

docs/eventing/broker/kafka-broker.md

@@ -105,6 +105,8 @@ The above `ConfigMap` is installed in the cluster. You can edit
 the configuration or create a new one with the same values
 depending on your needs.
 
+**NOTE:** The `default.topic.replication.factor` value must be less than or equal to the number of Kafka broker instances in your cluster. For example, if you only have one Kafka broker, the `default.topic.replication.factor` value should not be more than `1`.
+
 ## Set as default broker implementation
 
 To set the Kafka broker as the default implementation for all brokers in the Knative deployment,