Custom operation: enhancing the visibility of an important remark (#589)

* Custom operation: enhancing the visibility of an important remark

The information about necessity that the `__invoke()` method’s parameter must be called was easy to miss in the large amount of information. This proposal makes it more visible by putting it in its own paragraph and by writing it in bold.

* Update operations.md

Author: Deuchnord

core/operations.md

@@ -561,7 +561,8 @@ This action will be automatically registered as a service (the service name is t
 
 API Platform automatically retrieves the appropriate PHP entity using the data provider then deserializes user data in it,
 and for `POST` and `PUT` requests updates the entity with data provided by the user.
-By convention, the action's parameter must be called `$data`.
+
+**Warning: the `__invoke()` method parameter [MUST be called `$data`](https://symfony.com/doc/current/routing/extra_information.html)**, otherwise, it will not be filled correctly!
 
 Services (`$myService` here) are automatically injected thanks to the autowiring feature. You can type-hint any service
 you need and it will be autowired too.