I don't think this section is relevant, because I don't think Symfony ever looks for the descriptor inside the helper set. The descriptors are only ever used directly, for example:

* https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Console/Command/HelpCommand.php#L83
* https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Command/ContainerDebugCommand.php#L115

So you don't register it, I think you just simply create one and use it.