Document making lazy controller services by overriding `getSubscribedServices()`

[weaverryan]

It's a bit more advanced, but for truly lazy services in a controller, you can override getSubscribedServices, call the parent method, and add your own services that you want added to the service locator.

[nicolas-grekas]

For 4.2, we should also hint about ServiceSubscriberTrait since it should make configuring this locator easier (see https://github.com/symfony/symfony-docs/pull/9809/files)