Clarify that route defaults don't need a placeholder

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

@iamdto With the current wording I would add the comma as @WouterJ suggested. There has been and "and" and not a "but" before, hasn't it?