Mention exception for generic type aliases (#5478)

Author: asottile

docs/source/generics.rst

@@ -653,6 +653,8 @@ Generic protocols can also be recursive. Example:
 
    result = last(L())  # Inferred type of 'result' is 'int'
 
+.. _generic-type-aliases:
+
 Generic type aliases
 ********************
 

docs/source/kinds_of_types.rst

@@ -546,7 +546,8 @@ assigning the type to a variable:
 .. note::
 
     A type alias does not create a new type. It's just a shorthand notation for
-    another type -- it's equivalent to the target type.
+    another type -- it's equivalent to the target type except for
+    :ref:`generic aliases <generic-type-aliases>`.
 
 .. _named-tuples: