Document how to obtain a 404

Author: greg0ire

core/data-providers.md

@@ -95,7 +95,7 @@ final class BlogPostItemDataProvider implements ItemDataProviderInterface
           throw new ResourceClassNotSupportedException();
         }
 
-        // Retrieve the blog post item from somewhere
+        // Retrieve the blog post item from somewhere then return it or null if not found
         return new BlogPost($id);
     }
 }