Add comment to mqtt sensor last_reset_value_template that `state_clas…

…s` option must be `total` (home-assistant#31844)

source/_integrations/sensor.mqtt.markdown

@@ -149,7 +149,7 @@ json_attributes_topic:
   required: false
   type: string
 last_reset_value_template:
-  description: "Defines a [template](/docs/configuration/templating/#using-templates-with-the-mqtt-integration) to extract the last_reset. Available variables: `entity_id`. The `entity_id` can be used to reference the entity's attributes."
+  description: "Defines a [template](/docs/configuration/templating/#using-templates-with-the-mqtt-integration) to extract the last_reset. When `last_reset_value_template` is set, the `state_class` option must be `total`. Available variables: `entity_id`. The `entity_id` can be used to reference the entity's attributes."
   required: false
   type: template
 name: