docs: do not recomment to use PHP function

It is a bad practice.

Author: kenjis

user_guide_src/source/libraries/sessions.rst

@@ -353,12 +353,13 @@ destroy()
 ---------
 
 To clear the current session (for example, during a logout), you may
-simply use either PHP's `session_destroy() <https://www.php.net/session_destroy>`_
-function, or the library's ``destroy()`` method. Both will work in exactly the
-same way:
+simply use the library's ``destroy()`` method:
 
 .. literalinclude:: sessions/037.php
 
+This method will work in exactly the same way as PHP's
+`session_destroy() <https://www.php.net/session_destroy>`_ function.
+
 .. note:: This must be the last session-related operation that you do
     during the same request. All session data (including flashdata and
     tempdata) will be destroyed permanently and functions will be

user_guide_src/source/libraries/sessions/037.php

@@ -1,5 +1,3 @@
 <?php
 
-session_destroy();
-// or
 $session->destroy();