remove wrong notice on locales in the session, add reference to the relevant cookbook article instead

Author: xabbuh

book/translation.rst

@@ -520,15 +520,13 @@ via the ``request`` object::
 
     $request->setLocale('en_US');
 
-.. index::
-   single: Translations; Fallback and default locale
-
-It is also possible to store the locale in the session instead of on a per
-request basis. If you do this, each subsequent request will have this locale.
+.. tip::
 
-.. code-block:: php
+    Read :doc:`/cookbook/session/locale_sticky_session` to learn, how to store
+    the user's locale in the session.
 
-    $this->get('session')->set('_locale', 'en_US');
+.. index::
+   single: Translations; Fallback and default locale
 
 See the :ref:`book-translation-locale-url` section below about setting the
 locale via routing.