docs: improve expression

Author: kenjis

user_guide_src/source/general/environments.rst

@@ -22,7 +22,7 @@ By default, CodeIgniter comes with the ``ENVIRONMENT`` constant set to use
 the value provided in ``$_SERVER['CI_ENVIRONMENT']``, otherwise defaulting to
 ``production``. This can be set in several ways depending on your server setup.
 
-.. note:: The environment ``testing`` is the special one for PHPUnit testing.
+.. note:: The environment ``testing`` is reserved for PHPUnit testing.
     It has special conditions built into the framework at various places to assist with that.
     You can't use it for your development.