docs: add upgrading guide

Author: kenjis

user_guide_src/source/installation/upgrade_440.rst

@@ -15,6 +15,19 @@ Please refer to the upgrade instructions corresponding to your installation meth
 Breaking Changes
 ****************
 
+When you extend Exceptions
+==========================
+
+If you are extending ``CodeIgniter\Debug\Exceptions`` and have not overridden
+the ``exceptionHandler()`` method, defining the new ``Config\Exceptions::handler()``
+method in your **app/Config/Exceptions.php** will cause the specified Exception
+Handler to be executed.
+
+Your overridden code will no longer be executed, so make any necessary changes
+by defining your own exception handler.
+
+See :ref:`custom-exception-handlers` for the detail.
+
 Mandatory File Changes
 **********************
 
@@ -52,7 +65,9 @@ and it is recommended that you merge the updated versions with your application:
 Config
 ------
 
-- @TODO
+- app/Config/Exceptions.php
+    - Added the new method ``handler()`` that define custom Exception Handlers.
+      See :ref:`custom-exception-handlers`.
 
 All Changes
 ===========