docs(parser): improve documentation with Pydantic best practices (aws-powertools#5925)

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

* docs: improve the doc from PR

Co-authored-by: Leandro Damascena <[email protected]>
Signed-off-by: Alpha <[email protected]>

---------

Signed-off-by: Alpha <[email protected]>
Co-authored-by: Leandro Damascena <[email protected]>

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 "If you are still using the deprecated `root_validator` function, switch to `model_validator` for the latest Pydantic functionality."
+
 ```python title="model_validator.py" hl_lines="1 12-17"
 --8<-- "examples/parser/src/model_validator.py"
 ```