docs: add a note for typing_extensions

docs/usage/types.md

@@ -408,12 +408,21 @@ _(This script is complete, it should run "as is")_
 ## Annotated Types
 
 ### NamedTuple
+
 ```py
 {!.tmp_examples/annotated_types_named_tuple.py!}
 ```
 _(This script is complete, it should run "as is")_
 
 ### TypedDict
+
+!!! note
+    This is a new feature of the python standard library as of python 3.8.
+    Prior to python 3.8, it requires the [typing-extensions](https://pypi.org/project/typing-extensions/) package.
+    But required and optional fields are properly differentiated only since python 3.9.
+    It is hence recommanded using [typing-extensions](https://pypi.org/project/typing-extensions/) with python 3.8 as well.
+
+
 ```py
 {!.tmp_examples/annotated_types_typed_dict.py!}
 ```