Add another paragraph with functionalities not described yet.

en/tutorials/working-with-indexed-associations.rst

@@ -285,6 +285,10 @@ The same applies to DQL queries: The ``indexBy`` configuration acts as implicit
 
     echo $stock->getSymbol(); // will print "AAPL"
 
+If you want to use ``INDEX BY`` explicitly on an indexed association you are free to do so. Additionally
+indexed associations also work with the ``Collection::slice()`` functionality, no matter if marked as
+LAZY or EXTRA_LAZY.
+
 Outlook into the Future
 ~~~~~~~~~~~~~~~~~~~~~~~