Make retain optional.

docs/information/configuration.md

@@ -131,12 +131,10 @@ Normally devices and groups are specified in the `configuration.yaml`, example:
 devices:
   '0x00158d0001d82999':
     friendly_name: 'my_occupancy_sensor'
-    retain: true
 
 groups:
   '1':
     friendly_name: group_1
-    retain: true
 ```
 
 In case you want to specify devices and groups in a separate file, the following configuration can be used (equivalent of the above example).
@@ -151,14 +149,12 @@ groups: groups.yaml
 ```yaml
 '0x00158d0001d82999':
     friendly_name: 'my_occupancy_sensor'
-    retain: true
 ```
 
 **groups.yaml**
 ```yaml
 '1':
     friendly_name: group_1
-    retain: true
 ```
 
 ### Specifying MQTT user/password and network_key in a different file
@@ -186,7 +182,7 @@ The `configuration.yaml` allows to set device specific configuration. The follow
 
 ### All devices
 * `friendly_name`: Used in the MQTT topic of a device. By default this is the device ID (e.g. `0x00128d0001d9e1d2`).
-* `retain`: Retain MQTT messages of this device.
+* `retain`: Retain MQTT messages of this device (default `false`).
 * `qos`: QoS level for MQTT messages of this device. [What is QoS?](https://www.npmjs.com/package/mqtt#about-qos)
 * `homeassistant`: Allows to override values of the Home Assistant discovery payload. See example below.
 * `debounce`: Debounces messages of this device. When setting e.g. `debounce: 1` and a message from a device is received, zigbee2mqtt will not immediately publish this message but combine it with other messages received in that same second of that device. This is handy for e.g. the `WSDCGQ11LM` which publishes humidity, temperature and pressure at the same time but as 3 different messages.