Modernizing guide: Mention TypeGuard/TypeIs (#1887)

Author: srittau

docs/guides/modernizing.rst

@@ -341,3 +341,16 @@ Update your imports to use the new locations::
     from collections.abc import Hashable, Sized
 
     def f(x: Hashable) -> Sized: ...
+
+``typing.TypeGuard``
+====================
+
+*Available since:* Python 3.13, typing-extensions
+
+:data:`TypeIs <typing.TypeIs>` is an alternative to
+:data:`TypeGuard <typing.TypeGuard>` that usually has more intuitive
+behavior, but has other restrictions. See the documentation for
+:data:`TypeIs <typing.TypeIs>` for more information.
+
+Review existing uses of :data:`TypeGuard <typing.TypeGuard>` to see if they
+should be replaced with :data:`TypeIs <typing.TypeIs>`.