[Workflow] Document the fact workflows are tagged

[lyrixx]

Example usage:

use Symfony\Component\DependencyInjection\ServiceLocator;
use Symfony\Component\DependencyInjection\Attribute\AutowireLocator;
use Symfony\Component\Workflow\Attribute\AsTransitionListener;
use Symfony\Component\Workflow\Event\TransitionEvent;

class GenericListener
{
   public function __construct(
       #[AutowireLocator('workflow', 'name')]
       private ServiceLocator $workflows
   ) {
   }

   #[AsTransitionListener()]
   public function doSomething(TransitionEvent $event): void
   {
       $workflow = $this->workflows->get($event->getWorkflowName());
   }
}

iterator works as well

[OskarStark]

@lyrixx open up for a docs PR? 😄