Clarify that route defaults don't need a placeholder

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3960

> I think it's too early here, because we haven't talked about how wildcards map to arguments.

I don't fully agree with this. This is already discussed in the (previous) Controller chapter at [Route Parameters as Controller Arguments](http://symfony.com/doc/current/book/controller.html#route-parameters-as-controller-arguments) and at the beginning of this chapter at [Routing in Action](http://symfony.com/doc/current/book/routing.html#routing-in-action), if that's what you mean?

But anyway I agree that this may not be the best place to put this tip (I was not sure when I wrote it). I'll write a new cookbook as you suggested.

Thanks  for the feedback!