[WIP] Clarify that route defaults don't need a placeholder

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

But if we add XML support for it (using the same mechanism as used by DI), you think it would be a good idea?
Tbh, I'm not sure if I like the idea of array defaults (apart from the consistency). That's why I try to get other opinions