add warning to feature descriptions being incompatible with Symfony 2.3 and earlier

Since the documentation on symfony.com renders the same SensioFrameworkExtraBundle documentation for each Symfony version, there are descriptions visible for Symfony versions prior to 2.4 that cannot be used (SensioFrameworkExtraBundle 3.0 and newer require Symfony 2.4 or higher).

This was reported to the Symfony repository (see symfony/symfony#11599).