docs: add about Validation result changes in Upgrading

Author: kenjis

user_guide_src/source/installation/upgrade_416.rst

@@ -0,0 +1,55 @@
+#############################
+Upgrading from 4.1.5 to 4.1.6
+#############################
+
+.. contents::
+    :local:
+    :depth: 2
+
+Breaking Changes
+****************
+
+Validation result changes
+=========================
+
+The previous version of the Validation can't handle an array item.
+Because of the bug fix, the validation results may be different,
+or raise an TypeError.
+But the previous version's results are probably incorrect.
+
+And the Validation separated the validation process of multiple field
+like ``contacts.*.name`` and single field.
+When a single field has an array data, the previous version validates each element of the array.
+The validation rule gets an element of the array as the parameter.
+On the other hand, the current version passes the array to the validation rule as a whole.
+
+Breaking Enhancements
+*********************
+
+Project Files
+*************
+
+Numerous files in the project space (root, app, public, writable) received updates. Due to
+these files being outside of the system scope they will not be changed without your intervention.
+There are some third-party CodeIgniter modules available to assist with merging changes to
+the project space: `Explore on Packagist <https://packagist.org/explore/?query=codeigniter4%20updates>`_.
+
+.. note:: Except in very rare cases for bug fixes, no changes made to files for the project space
+    will break your application. All changes noted here are optional until the next major version,
+    and any mandatory changes will be covered in the sections above.
+
+Content Changes
+===============
+
+The following files received significant changes (including deprecations or visual adjustments)
+and it is recommended that you merge the updated versions with your application:
+
+*
+
+All Changes
+===========
+
+This is a list of all files in the project space that received changes;
+many will be simple comments or formatting that have no effect on the runtime:
+
+*

user_guide_src/source/installation/upgrading.rst

@@ -8,6 +8,7 @@ upgrading from.
 .. toctree::
     :titlesonly:
 
+    Upgrading from 4.1.5 to 4.1.6 <upgrade_416>
     Upgrading from 4.1.4 to 4.1.5 <upgrade_415>
     Upgrading from 4.1.3 to 4.1.4 <upgrade_414>
     Upgrading from 4.1.2 to 4.1.3 <upgrade_413>