[docs] Add example of using @final with TypedDict

Fixes python#18543

This is a useful but also non-obvious pattern that seems worthy of a docs example :)

Author: magicmark

docs/source/typed_dict.rst

@@ -289,6 +289,32 @@ need to give each TypedDict the same key where each value has a unique
 :ref:`Literal type <literal_types>`. Then, check that key to distinguish
 between your TypedDicts.
 
+You can also use :py:class:`@final <https://docs.python.org/3/library/typing.html#typing.final>` to indicate that an
+instance of TypedDict will not be reassigned or subclassed. This allows mypy to do type refinement based on unique
+properties of the TypedDict:
+
+.. code-block:: python
+
+    from typing import TypedDict, final
+
+    @final
+    class Movie(TypedDict):
+        director: str
+        runtime: int
+
+    @final
+    class Book(TypedDict):
+        author: str
+        pages: int
+
+    def foo(movie_or_book: Movie | Book) -> None:
+        if 'director' in movie_or_book:
+            director = movie_or_book['director']
+            runtime = movie_or_book['runtime']
+        elif 'author' in movie_or_book:
+            author = movie_or_book['author']
+            pages = movie_or_book['pages']
+
 Inline TypedDict types
 ----------------------