Clarify that route defaults don't need a placeholder

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

How about:

>
In fact, you can use the `defaults` array to specify extra parameters that will
then be accessible as arguments to your controller:

I don't think that most users really know what "request attributes" are :).