docs: add the reason for the change

Co-authored-by: Michal Sniatala <[email protected]>

Author: kenjis

user_guide_src/source/changelogs/v4.1.5.rst

@@ -14,7 +14,7 @@ BREAKING
 
 - Fixed `a bug <https://github.com/codeigniter4/CodeIgniter4/issues/2913>`_ on CSRF protection. Now CSRF protection works on PUT/PATCH/DELETE requests when CSRF filter is applied. If you use such requests, you need to send CSRF token.
 - In the previous version, if you didn't provide your own headers, ``CURLRequest`` would send the request-headers from the browser, due to a bug. As of this version, it does not send them.
-- Fixed ``BaseBuilder::insertBatch()`` return value. Now it returns SQL string array instead of wrong affected row number when testMode.
+- Fixed ``BaseBuilder::insertBatch()`` return value for ``testMode``. Now it returns SQL string array instead of a number of affected rows. This change was made because of maintaining compatibility between returning types for batch methods. Now the returned data type for ``BaseBuilder::insertBatch()`` is the same as the `updateBatch()` method.
 - Major optimizations have been made to the way data is processed in ``BaseBuilder::insertBatch()`` and ``BaseBuilder::updateBatch()`` methods. This resulted in reduced memory usage and faster query processing. As a trade-off, the result generated by the ``$query->getOriginalQuery()`` method was changed. It no longer returns the query with the binded parameters, but the actual query that was run.
 
 Enhancements