docs: improve the doc for the customer from v2 library.

Author: AlphaWong

docs/utilities/parser.md

@@ -259,6 +259,8 @@ If you run using a test event `{"message": "hello universe"}` you should expect
 
 `model_validator` can help when you have a complex validation mechanism. For example finding whether data has been omitted or comparing field values.
 
+???+ tip For users of `root_validator` (deprecated as of version 3.0.0), it is recommended to use `model_validator` instead.
+
 ```python title="model_validator.py" hl_lines="1 12-17"
 --8<-- "examples/parser/src/model_validator.py"
 ```