[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

Array parameters do not work in all cases. It is not supported for Apache rewrite rules (which is deprecated anyway) and, as you figured out, in XML definitions. So as long as it's not consistently working, I would not advertise it.