Add mqtt.dump docs (#11955)

source/_docs/mqtt/service.markdown

@@ -37,7 +37,7 @@ payload_template: {{ states('device_tracker.paulus') }}
 ```yaml
 topic: home-assistant/light/1/state
 payload: "{\"Status\":\"off\", \"Data\":\"something\"}"
-``` 
+```
 
 Example of how to use `qos` and `retain`:
 
@@ -47,3 +47,16 @@ payload: on
 qos: 2
 retain: true
 ```
+
+### Service `mqtt.dump`
+
+Listen to the specified topic matcher and dumps all received messages within a specific duration into the file `mqtt_dump.txt` in your config folder. This is useful when debugging a problem.
+
+| Service data attribute | Optional | Description |
+| ---------------------- | -------- | ----------- |
+| `topic` | no | Topic to dump. Can contain a wildcard (`#` or `+`).
+| `duration` | no | Duration in seconds that we will listen for messages. Default is 5 seconds.
+
+```yaml
+topic: openzwave/#
+```

source/_integrations/mqtt.markdown

@@ -44,7 +44,7 @@ This allows you to connect to the MQTT broker with user `homeassistant` and pass
 
 - [Certificate](/docs/mqtt/certificate/)
 - [Discovery](/docs/mqtt/discovery/)
-- [Publish service](/docs/mqtt/service/)
+- [Publish & Dump services](/docs/mqtt/service/)
 - [Birth and last will messages](/docs/mqtt/birth_will/)
 - [Testing your setup](/docs/mqtt/testing/)
 - [Logging](/docs/mqtt/logging/)