docs: add changelog and upgrade guide

Author: kenjis

user_guide_src/source/changelogs/v4.3.7.rst

@@ -15,6 +15,9 @@ BREAKING
 - **FeatureTestTrait:** When using :ref:`withBodyFormat() <feature-formatting-the-request>`,
   the priority of the request body has been changed.
   See :ref:`Upgrading Guide <upgrade-437-feature-testing>` for details.
+- **Validation:** The return value of ``Validation::loadRuleGroup()`` has been
+  changed from "**rules array**" to "**array** of **rules array** and **customErrors array**"
+  (``[rules, customErrors]``).
 
 Message Changes
 ***************

user_guide_src/source/installation/upgrade_437.rst

@@ -39,6 +39,19 @@ is not used::
 
 Previously, the ``$body`` was used for the request body.
 
+Return value of Validation::loadRuleGroup()
+===========================================
+
+The return value of ``Validation::loadRuleGroup()`` has been changed from
+"**rules array**" to "**array** of **rules array** and **customErrors array**"
+(``[rules, customErrors]``).
+
+If you use the method, update the code like the following::
+
+    $rules = $this->validation->loadRuleGroup($rules);
+        ↓
+    [$rules, $customErrors] = $this->validation->loadRuleGroup($rules);
+
 Breaking Enhancements
 *********************