bpo-45680: typing docs: improve links to docs on GenericAlias/__class_getitem__ (GH-29387)

Author: AlexWaygood

Doc/library/typing.rst

@@ -49,8 +49,8 @@ annotations. These include:
      *Introducing* :class:`Protocol` and the
      :func:`@runtime_checkable<runtime_checkable>` decorator
 * :pep:`585`: Type Hinting Generics In Standard Collections
-     *Introducing* the ability to use builtin collections and ABCs as
-     :term:`generic types<generic type>`
+     *Introducing* :class:`types.GenericAlias` and the ability to use standard
+     library classes as :ref:`generic types<types-genericalias>`
 * :pep:`586`: Literal Types
      *Introducing* :data:`Literal`
 * :pep:`589`: TypedDict: Type Hints for Dictionaries with a Fixed Set of Keys
@@ -291,8 +291,8 @@ A user-defined class can be defined as a generic class.
 single type parameter ``T`` . This also makes ``T`` valid as a type within the
 class body.
 
-The :class:`Generic` base class defines :meth:`__class_getitem__` so that
-``LoggedVar[t]`` is valid as a type::
+The :class:`Generic` base class defines :meth:`~object.__class_getitem__` so
+that ``LoggedVar[t]`` is valid as a type::
 
    from collections.abc import Iterable
 

Misc/NEWS.d/next/Documentation/2021-11-03-14-51-03.bpo-45680.9_NTFU.rst

@@ -0,0 +1,3 @@
+Amend the docs on ``GenericAlias`` objects to clarify that non-container
+classes can also implement ``__class_getitem__``. Patch contributed by Alex
+Waygood.