docs: improve transforming records example

Author: heitorlessa

docs/utilities/data_classes.md

@@ -985,10 +985,13 @@ To do that, you can use `KinesisFirehoseDataTransformationResponse` class along
 
 === "Transforming streaming records"
 
-    ```python
+    ```python hl_lines="2-3 12 28"
     --8<-- "examples/event_sources/src/kinesis_firehose_delivery_stream.py"
     ```
 
+    1. **Ingesting JSON payloads?** <br><br> Use `record.data_as_json` to easily deserialize them.
+    2. For your convenience, `base64_from_json` serializes a dict to JSON, then encode as base64 data.
+
 === "Dropping invalid records"
 
     ```python

examples/event_sources/src/kinesis_firehose_delivery_stream.py

@@ -13,14 +13,13 @@ def lambda_handler(event: KinesisFirehoseEvent, context: LambdaContext):
 
     for record in event.records:
         # get original data using data_as_text property
-        data = record.data_as_text
+        data = record.data_as_text  # (1)!
 
-        ## do all kind of stuff with data
         ## generate data to return
         transformed_data = {"new_data": "transformed data using Powertools", "original_payload": data}
 
         processed_record = record.build_data_transformation_response(
-            data=base64_from_json(transformed_data),
+            data=base64_from_json(transformed_data),  # (2)!
         )
 
         result.add_record(processed_record)