Expand documentation of 'qos' configuration in mqtt_consumer input (#4784)

etc/telegraf.conf

@@ -3918,8 +3918,15 @@
 #   ## schema can be tcp, ssl, or ws.
 #   servers = ["tcp://localhost:1883"]
 #
-#   ## MQTT QoS, must be 0, 1, or 2
+#   ## QoS policy for messages
+#   ##   0 = at most once
+#   ##   1 = at least once
+#   ##   2 = exactly once
+#   ##
+#   ## When using a QoS of 1 or 2, you should enable persistent_session to allow
+#   ## resuming unacknowledged messages.
 #   qos = 0
+#
 #   ## Connection timeout for initial connection in seconds
 #   connection_timeout = "30s"
 #