[DOC]: Document async_contract_fulfillment enum.

[ahendriksen]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

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

cccl/libcudacxx/include/cuda/std/detail/libcxx/include/__cuda/barrier.h

Line 49 in ea33ba9

enum async_contract_fulfillment

cccl/libcudacxx/include/cuda/std/detail/libcxx/include/__cuda/barrier.h

Line 865 in ea33ba9

async_contract_fulfillment memcpy_async(_Group const & __group, void * __destination, void const * __source, aligned_size_t<_Alignment> __size, barrier<_Sco, _CompF> & __barrier) {