bpo-42198: Link to GenericAlias in typing and expressions (GH-23030)



Follow up to 7cdf30fff39ea97f403b5472096349998d190e30 and 4173320920706b49a004bdddd8d7108e8984e3fc. This addresses the point "1. Update links in typing, subscription and union to point to GenericAlias." in the bpo for this PR.
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 938a973..512aa5a 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -837,14 +837,8 @@
 A string's items are characters.  A character is not a separate data type but a
 string of exactly one character.
 
-..
-   At the time of writing this, there is no documentation for generic alias
-   or PEP 585.  Thus the link currently points to PEP 585 itself.
-   Please change the link for generic alias to reference the correct
-   documentation once documentation for PEP 585 becomes available.
-
 Subscription of certain :term:`classes <class>` or :term:`types <type>`
-creates a `generic alias <https://www.python.org/dev/peps/pep-0585/>`_.
+creates a :ref:`generic alias <types-genericalias>`.
 In this case, user-defined classes can support subscription by providing a
 :meth:`__class_getitem__` classmethod.