[Milvus] <docs>: add more comments to externalKafka parameter

Signed-off-by: Wang Yanfeng <wyfeng001@gmail.com>

charts/milvus/values.yaml

@@ -901,6 +901,18 @@ externalPulsar:
 ###################################
 # External kafka
 # - these configs are only used when `externalKafka.enabled` is true
+# - note that the following are just examples, you should confirm the 
+#   value of brokerList and mechanisms according to the actual external 
+#   Kafka configuration. E.g. If you select the AWS MSK, the configuration
+#   should look something like this:
+#   externalKafka:
+#     enabled: true
+#     brokerList: "xxxx:9096"
+#     securityProtocol: SASL_SSL
+#     sasl:
+#       mechanisms: SCRAM-SHA-512
+#       password: "xxx"
+#       username: "xxx"
 ###################################
 externalKafka:
   enabled: false