docs: add user guide about breaking changes of previous_url

Author: tearoom6

user_guide_src/source/changelogs/v4.2.0.rst

@@ -32,6 +32,7 @@ Behavior Changes
 - To prevent unexpected access from the web browser, if a controller is added to a cli route (``$routes->cli()``), all methods of that controller are no longer accessible via auto-routing.
 - There is a possible backward compatibility break for those users extending the History Collector and they should probably update ``History::setFiles()`` method.
 - The :php:func:`dot_array_search`'s unexpected behavior has been fixed. Now ``dot_array_search('foo.bar.baz', ['foo' => ['bar' => 23]])`` returns ``null``. The previous versions returned ``23``.
+- The ``CodeIgniter::storePreviousURL()`` has been changed to store only the URLs whose Content-Type was ``text/html``. It also affects the behavior of ``previous_url()`` and ``redirect()->back()``.
 
 Enhancements
 ************

user_guide_src/source/installation/upgrade_420.rst

@@ -42,6 +42,7 @@ Breaking Changes
 ****************
 
 - The ``system/bootstrap.php`` file no longer returns a ``CodeIgniter`` instance, and does not load the ``.env`` file (now handled in ``index.php`` and ``spark``). If you have code that expects these behaviors it will no longer work and must be modified. This has been changed to make `Preloading <https://www.php.net/manual/en/opcache.preloading.php>`_ easier to implement.
+- ``previous_url()`` has been changed to return only the URLs whose Content-Type was ``text/html``. Accordingly, the behavior of ``redirect()->back()`` has been changed.
 
 Breaking Enhancements
 *********************