Clarify runtime implementation of Generic (#1876)

Author: Viicos

docs/spec/generics.rst

@@ -139,8 +139,8 @@ for variance, see below).
 takes a single type parameter ``T``. This also makes ``T`` valid as
 a type within the class body.
 
-The ``Generic`` base class uses a metaclass that defines ``__getitem__``
-so that ``LoggedVar[t]`` is valid as a type::
+The ``Generic`` base class implements the :func:`~object.__class_getitem__`
+classmethod so that ``LoggedVar[t]`` is valid as a type::
 
   from collections.abc import Iterable