title: Custom GET operation for API platform generates wrong documentation
body: <p>I have added the following operation under TeachingClass entity.</p>
  
  <pre><code>App\Entity\TeachingClass:
      collectionOperations:
          # ...
      itemOperations:
          # ...
  
          get_learning_skills:
              method: GET
              path: /auth/v1/teaching-class/{id}/learning-skills
              resourceClass: 'App\Entity\LearningSkill' # Doesn't seem to work
              controller: App\Controller\Api\LearningSkillApiController
              normalization_context:
                  groups: ['learning_skill_list']
              security: 'is_granted("HAS_TEACHING_CLASS_ACCESS", object)'
              swagger_context:
                  summary: "Retrieves the collection of LearningSkill resources belonging to a specific TeachingClass."
                  description: "LearningSkills belonging to a specific TeachingClass"
  </code></pre>
  
  <p>The end-point correctly returns a collection of LearningSkill entities by the configured controller:</p>
  
  <pre class="lang-php prettyprint-override"><code>&lt;?php
  
  namespace App\Controller\Api;
  
  use App\Entity\LearningSkill;
  use App\Entity\TeachingClass;
  use App\Repository\LearningSkillRepository;
  use Symfony\Bundle\FrameworkBundle\Controller\Controller;
  use Symfony\Component\HttpFoundation\JsonResponse;
  
  /**
   * Class LearningSkillApiController.
   */
  class LearningSkillApiController
  {
      private $learningSkillRepository;
  
      public function __construct(LearningSkillRepository $learningSkillRepository)
      {
          $this-&gt;learningSkillRepository = $learningSkillRepository;
      }
  
      public function __invoke(TeachingClass $data)
      {
          return $this-&gt;byTeachingClass($data);
      }
  
      private function byTeachingClass(TeachingClass $teachingClass)
      {
          return $this-&gt;learningSkillRepository-&gt;findByTeachingClass($teachingClass);
      }
  }
  </code></pre>
  
  <p>However, my problem is that the generated API doc is wrong:</p>
  
  <p><a href="https://i.stack.imgur.com/DiQGp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DiQGp.png" alt="API Platform generated documentation"></a></p>
  
  <p>How do I make the documentation reflect that the response is a collection of LearningSkill entities (instead of a TeachingClass entity)?</p>