docs: add changelog and upgrade guide

Author: kenjis

user_guide_src/source/changelogs/v4.3.6.rst

@@ -12,6 +12,28 @@ Release Date: Unreleased
 BREAKING
 ********
 
+Interface Changes
+=================
+
+.. note:: As long as you have not extended the relevant CodeIgniter core classes
+    or implemented these interfaces, all these changes are backward compatible
+    and require no intervention.
+
+ValidationInterface::check()
+----------------------------
+
+- The second parameter has changed from ``string $rule`` to ``$rules``.
+- The optional fourth parameter ``$dbGroup = null`` has been added.
+
+Method Signature Changes
+========================
+
+Validation::check()
+-------------------
+
+- The second parameter has changed from ``string $rule`` to ``$rules``.
+- The optional fourth parameter ``$dbGroup = null`` has been added.
+
 Message Changes
 ***************
 
@@ -24,6 +46,11 @@ Deprecations
 Bugs Fixed
 **********
 
+- **Validation:** Fixed a bug that ``$DBGroup`` is ignored when checking
+  the value of a placeholder.
+- **Validation:** Fixed a bug that ``check()`` cannot specify non-default
+  database group.
+
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_
 for a complete list of bugs fixed.

user_guide_src/source/installation/upgrade_436.rst

@@ -21,6 +21,9 @@ Breaking Changes
 Breaking Enhancements
 *********************
 
+- The method signatures of ``ValidationInterface::check()`` and ``Validation::check()``
+  have been changed. If you implement or extend them, update the signatures.
+
 Project Files
 *************