[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

In [symfony/symfony#11171](https://github.com/symfony/symfony/issues/11171#issuecomment-46548031) @stloyd gave an example on how it's used in Sylius.

So I suggest that for the moment we leave it undocumented, open an issue on `symfony/symfony` for adding XML support for array defaults and once it's done we document it.

If you agree with this I'll take care of it.