docs: move method signature changes in one place

Author: kenjis

user_guide_src/source/changelogs/v4.2.0.rst

@@ -12,8 +12,8 @@ Release Date: Not Released
 BREAKING
 ********
 
-- The method signature of ``Validation::setRule()`` has been changed.
 - The method signature of ``CodeIgniter\Database\BaseBuilder::join()`` and ``CodeIgniter\Database\*\Builder::join()`` have been changed.
+- The method signature of ``Validation::setRule()`` has been changed. The ``string`` typehint on the ``$rules`` parameter was removed. Extending classes should likewise remove the parameter so as not to break LSP.
 - The method signature of ``CodeIgniter\CLI\CommandRunner::_remap()`` has been changed to fix a bug.
 - The ``CodeIgniter\CodeIgniter`` class has a new property ``$context`` and it must have the correct context at runtime. So the following files have been changed:
     - ``public/index.php``