Template filter is_defined missing documentation (#21459)

source/_docs/configuration/templating.markdown

@@ -508,6 +508,20 @@ The temperature is 25°C
 
 {% endraw %}
 
+### Is defined
+
+Sometimes a template should only return if a value or object is defined, if not, the supplied default value could be returned. This can be useful to validate a JSON payload.
+The `is_defined` filter allows to throw an error if a value or object is not defined.
+
+Example using `is_defined` to parse a JSON payload:
+
+```text
+{{ value_json.val | is_defined }}
+```
+
+This will throw an error `UndefinedError: 'value_json' is undefined` if the JSON payload has no `val` attribute.
+
+
 ### Distance
 
 Not supported in [limited templates](#limited-templates).

source/_integrations/sensor.mqtt.markdown

@@ -187,7 +187,7 @@ unit_of_measurement:
   required: false
   type: string
 value_template:
-  description: "Defines a [template](/docs/configuration/templating/#processing-incoming-data) to extract the value. Available variables: `entity_id`. The `entity_id` can be used to reference the entity's attributes."
+  description: "Defines a [template](/docs/configuration/templating/#processing-incoming-data) to extract the value. Available variables: `entity_id`. The `entity_id` can be used to reference the entity's attributes. If the template throws an error, the current state will be used instead."
   required: false
   type: template
 {% endconfiguration %}