MINOR: tweak the doc of "num.network.threads" (apache#13327)

Reviewers: Luke Chen <[email protected]>

core/src/main/scala/kafka/server/KafkaConfig.scala

@@ -709,7 +709,7 @@ object KafkaConfig {
   "start from " + MaxReservedBrokerIdProp + " + 1."
   val MessageMaxBytesDoc = TopicConfig.MAX_MESSAGE_BYTES_DOC +
     s"This can be set per topic with the topic level <code>${TopicConfig.MAX_MESSAGE_BYTES_CONFIG}</code> config."
-  val NumNetworkThreadsDoc = "The number of threads that the server uses for receiving requests from the network and sending responses to the network"
+  val NumNetworkThreadsDoc = s"The number of threads that the server uses for receiving requests from the network and sending responses to the network. Noted: each listener (except for controller listener) creates its own thread pool."
   val NumIoThreadsDoc = "The number of threads that the server uses for processing requests, which may include disk I/O"
   val NumReplicaAlterLogDirsThreadsDoc = "The number of threads that can move replicas between log directories, which may include disk I/O"
   val BackgroundThreadsDoc = "The number of threads to use for various background processing tasks"