gh-129567: Add a note to typing.TypedDict docs about name mangling

Author: sobolevn

Doc/library/typing.rst

@@ -2551,15 +2551,20 @@ types.
 
    This functional syntax allows defining keys which are not valid
    :ref:`identifiers <identifiers>`, for example because they are
-   keywords or contain hyphens::
+   keywords or contain hyphens, or when key names must not be mangled
+   like regular private names::
 
       # raises SyntaxError
       class Point2D(TypedDict):
           in: int  # 'in' is a keyword
           x-y: int  # name with hyphens
 
+      class Definition(TypedDict):
+          __schema: str  # mangled to `_Definition__schema`
+
       # OK, functional syntax
       Point2D = TypedDict('Point2D', {'in': int, 'x-y': int})
+      Definition = TypedDict('Definition', {'__schema': str})  # not mangled
 
    By default, all keys must be present in a ``TypedDict``. It is possible to
    mark individual keys as non-required using :data:`NotRequired`::