Clarify that route defaults don't need a placeholder

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

I think it's too early here, because we haven't talked about how wildcards map to arguments. We *are* talking about "defaults" here, but for a totally different purpose (making wildcards optional).

I'd like to see this as a tiny new cookbook entry called something like: "Passing extra Information from a Route to a Controller". We could add a little link to it at the bottom of the `Route Parameters and Controller Arguments` section.

What do you think?