[ci skip] Add upgrade instructions about the APC deprecation

user_guide_src/source/installation/upgrade_316.rst

@@ -12,3 +12,17 @@ Replace all files and directories in your *system/* directory.
 
 .. note:: If you have any custom developed files in these directories,
 	please make copies of them first.
+
+Step 2: Remove usage of the APC Cache driver (deprecation)
+==========================================================
+
+The :doc:`Cache Library <../libraries/caching>` APC driver is now
+deprecated, as the APC extension is effectively dead, as explained in its
+`PHP Manual page <https://secure.php.net/manual/en/intro.apc.php>`_.
+
+If your application happens to be using it, you can switch to another
+cache driver, as APC support will be removed in a future CodeIgniter
+version.
+
+.. note:: The driver is still available, but you're strongly encouraged
+	to remove its usage sooner rather than later.