Update docs about MQTT lambdas (#809)

Co-authored-by: Mateusz <mateusz@kijowski.info>

components/mqtt.rst

@@ -334,9 +334,15 @@ Configuration variables:
 
     This action can also be used in :ref:`lambdas <config-lambda>`:
 
+    .. code-block:: yaml
+
+        mqtt:
+          # Give the mqtt component an ID
+          id: mqtt_client
+
     .. code-block:: cpp
 
-        App.get_mqtt_client()->subscribe("the/topic", [=](const std::string &payload) {
+        id(mqtt_client).subscribe("the/topic", [=](const std::string &payload) {
             // do something with payload
         });
 
@@ -397,9 +403,15 @@ Configuration variables:
 
     This action can also be used in :ref:`lambdas <config-lambda>`:
 
+    .. code-block:: yaml
+
+        mqtt:
+          # Give the mqtt component an ID
+          id: mqtt_client
+
     .. code-block:: cpp
 
-        App.get_mqtt_client()->subscribe_json("the/topic", [=](JsonObject &root) {
+        id(mqtt_client).subscribe_json("the/topic", [=](JsonObject &root) {
             // do something with JSON-decoded value root
         });
 
@@ -522,6 +534,22 @@ the MQTT broker.
         then:
           - logger.log: MQTT is connected!
 
+.. note::
+
+    This action can also be written in :ref:`lambdas <config-lambda>`:
+
+    .. code-block:: yaml
+
+        mqtt:
+          # Give the mqtt component an ID
+          id: mqtt_client
+
+    .. code-block:: cpp
+
+        if (id(mqtt_client)->is_connected()) {
+          // do something if MQTT is connected
+        }
+
 See Also
 --------