docs: add the reason for the change

Co-authored-by: Michal Sniatala <michal@sniatala.pl>

Author: kenjis

user_guide_src/source/installation/upgrade_415.rst

@@ -67,7 +67,7 @@ Query Builder changes
 For optimization and a bug fix, the following behaviors, mostly used in testing, have been changed.
 
 - When you use ``insertBatch()`` and ``updateBatch()``, the return value of ``$query->getOriginalQuery()`` has changed. It no longer returns the query with the binded parameters, but the actual query that was run.
-- If ``testMode`` is ``true``, ``insertBatch()`` will return an SQL string array instead of the number of affected rows that were wrong.
+- If ``testMode`` is ``true``, ``insertBatch()`` will return an SQL string array instead of the number of affected rows. This change was made so that the returned data type is the same as the ``updateBatch()`` method.
 
 Breaking Enhancements
 =====================