[DOC]: Missing link to cuda::proclaim_copyable_arguments

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Is this for new documentation, or an update to existing docs?

Update

### Describe the incorrect/future/missing documentation

In the current API documentation of cuda/functional
`cuda::proclaim_copyable_arguments` is missing a hyperlink to its detailed description.

### If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

https://nvidia.github.io/cccl/unstable/libcudacxx/extended_api/functional.html