Merge pull request #2660 from choco14t/docs/graphql-field-middleware

docs(graphql): add warning section for field middleware

Author: kamilmysliwiec

content/graphql/field-middleware.md

@@ -43,6 +43,8 @@ Now whenever we request the `title` field of `Recipe` object type, the original
 
 > info **Hint** To learn how you can implement a field-level permissions system with the use of [extensions](/graphql/extensions) feature, check out this [section](/graphql/extensions#using-custom-metadata).
 
+> warning **Warning** Field middleware can be applied only to `ObjectType` classes. For more details, check out this [issue](https://github.com/nestjs/graphql/issues/2446).
+
 Also, as mentioned above, we can control the field's value from within the middleware function. For demonstration purposes, let's capitalise a recipe's title (if present):
 
 ```typescript