[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

Looking at the schema definition, this doesn't seem to be possible in XML at the moment. The [``default`` element](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Routing/Loader/schema/routing/routing-1.0.xsd#L29) can [only contain string values](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Routing/Loader/schema/routing/routing-1.0.xsd#L58-64).