Merge pull request #5317 from kenjis/fix-docs-upgrade_4xx.rst

docs: add about redirect() change

Author: kenjis

user_guide_src/source/installation/upgrade_4xx.rst

@@ -94,6 +94,9 @@ General Adjustments
 **Helpers**
 
 - Helpers are pretty much the same as before, though some have been simplified.
+- In CI4, ``redirect()`` returns a ``RedirectResponse`` instance instead of redirecting and terminating script execution. You must return it.
+    - `redirect() Documentation CodeIgniter 3.X <https://codeigniter.com/userguide3/helpers/url_helper.html#redirect>`_
+    - `redirect() Documentation CodeIgniter 4.X <../general/common_functions.html#redirect>`_
 
 **Events**