[DOC]: Document async_contract_fulfillment enum.

### 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

`cuda::memcpy_async` returns `cuda::async_contract_fulfillment`, but is documented to return `void`.

We should document the `async_contract_fulfillment` enum.

Brought up by: https://github.com/NVIDIA/cccl/pull/405/files#r1318506699

### 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/libcudacxx/extended_api/asynchronous_operations/memcpy_async.html

https://github.com/NVIDIA/cccl/blob/ea33ba965713ae647dd9de4814260fea345bee9e/libcudacxx/include/cuda/std/detail/libcxx/include/__cuda/barrier.h#L49

https://github.com/NVIDIA/cccl/blob/ea33ba965713ae647dd9de4814260fea345bee9e/libcudacxx/include/cuda/std/detail/libcxx/include/__cuda/barrier.h#L865