[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

I guess there may be use cases where structured default values may be useful (even if I can't think of an example instantly). Though I suspect people currently would something differently since the way you can use the defaults for this, is not documented. So, I'd suggest to add XML support.