docs: add about DB caching

Author: kenjis

user_guide_src/source/installation/upgrade_database.rst

@@ -14,6 +14,7 @@ Documentations
 What has been changed
 =====================
 - The functionality in CI3 is basically the same as in CI4.
+- `Database Caching <https://www.codeigniter.com/userguide3/database/caching.html>`_ in CI3 is removed.
 - The method names have changed to camelCase and the :doc:`Query Builder <../database/query_builder>`
   now needs to be initialized before you can run queries on it.
 
@@ -40,6 +41,8 @@ Upgrade Guide
     - ``$this->db->select_max('age');`` to ``$builder->selectMax('age');``
     - ``$this->db->join('comments', 'comments.id = blogs.id');`` to ``$builder->join('comments', 'comments.id = blogs.id');``
     - ``$this->db->having('user_id',  45);`` to ``$builder->having('user_id',  45);``
+6. CI4 does not provide `Database Caching <https://www.codeigniter.com/userguide3/database/caching.html>`_
+   in CI3, so if you need to cache the result, use :doc:`../libraries/caching` instead.
 
 Code Example
 ============