Add note to TypedDict.__total__docs

As discussed in comments to python#109544, the semantics of this attribute
are somewhat confusing. Add a note explaining its limitations and
steering users towards __required_keys__ and __optional_keys__ instead.

Doc/library/typing.rst

@@ -2404,6 +2404,13 @@ types.
          >>> Point3D.__total__
          True
 
+      This attribute reflects *only* the value of the ``total`` argument
+      to the current ``TypedDict`` class, not whether the class is semantically
+      total. For example, a ``TypedDict`` with ``__total__`` set to True may
+      have keys marked with :data:`NotRequired`, or it may inherit from another
+      ``TypedDict`` with ``total=False``. Therefore, it is generally better to use
+      :attr:`__required_keys__` and :attr:`__optional_keys__` for introspection.
+
    .. attribute:: __required_keys__
 
       .. versionadded:: 3.9