[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

Then I think it's safe to remove the array example from this PR (which is based on 2.3) and then add it back based on the master branch once yours get merged (as it will be considered a new feature).