docs: improve exception example

Author: heitorlessa

docs/utilities/data_classes.md

@@ -1002,10 +1002,12 @@ To do that, you can use `KinesisFirehoseDataTransformationResponse` class along
 
 === "Indicating a processing failure"
 
-    ```python
+    ```python hl_lines="2-3 33"
     --8<-- "examples/event_sources/src/kinesis_firehose_response_exception.py"
     ```
 
+    1. This record will now be sent to your [S3 bucket in the `processing-failed` folder](https://docs.aws.amazon.com/firehose/latest/dev/data-transformation.html#data-transformation-failure-handling){target="_blank"}.
+
 ### Lambda Function URL
 
 === "app.py"

examples/event_sources/src/kinesis_firehose_response_exception.py

@@ -2,21 +2,24 @@
     KinesisFirehoseDataTransformationRecord,
     KinesisFirehoseDataTransformationResponse,
     KinesisFirehoseEvent,
+    event_source,
 )
 from aws_lambda_powertools.utilities.serialization import base64_from_json
 from aws_lambda_powertools.utilities.typing import LambdaContext
 
 
+@event_source(data_class=KinesisFirehoseEvent)
 def lambda_handler(event: dict, context: LambdaContext):
     firehose_event = KinesisFirehoseEvent(event)
     result = KinesisFirehoseDataTransformationResponse()
 
     for record in firehose_event.records:
         try:
             payload = record.data_as_text  # base64 decoded data as str
-            ## do all kind of stuff with payload
-            ## generate data to return
+
+            # generate data to return
             transformed_data = {"tool_used": "powertools_dataclass", "original_payload": payload}
+
             # Default result is Ok
             processed_record = KinesisFirehoseDataTransformationRecord(
                 record_id=record.record_id,
@@ -27,7 +30,7 @@ def lambda_handler(event: dict, context: LambdaContext):
             processed_record = KinesisFirehoseDataTransformationRecord(
                 record_id=record.record_id,
                 data=record.data,
-                result="ProcessingFailed",
+                result="ProcessingFailed",  # (1)!
             )
 
         result.add_record(processed_record)