Clarify that route defaults don't need a placeholder

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

Ah, you're right - I didn't see that we already explained the arguments part in "Routing in Action". So, it's not too early here. I still also like the idea of a separate cookbook entry, but I'm more open to *where* we put that link to it :).