docs: add upgrade_440

Author: kenjis

user_guide_src/source/installation/upgrade_440.rst

@@ -50,9 +50,12 @@ If your code depends on this bug, fix the segment number.
 Site URI Changes
 ================
 
-When your baseURL has sub-directories and you get the relative path to baseURL of
-the current URI by the ``URI::getPath()`` method, you must use the new
-``SiteURI::getRoutePath()`` method instead.
+- Because of the rework for the current URI determination, the framework may return
+  site URIs or the URI paths slightly differently than in previous versions. It may
+  break your test code. Update assertions if the existing tests fail.
+- When your baseURL has sub-directories and you get the relative path to baseURL of
+  the current URI by the ``URI::getPath()`` method, you must use the new
+  ``SiteURI::getRoutePath()`` method instead.
 
 See :ref:`v440-site-uri-changes` for details.