[DOC]: [CUB] document best practices for operators

### 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

It would be nice to have a section in the device wide primitives documentation for CUB that mentions the following:

1) Using a functor vs using a lambda
2) Passing arguments in by reference or value

### 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.

_No response_