Document why combining pipeline<thread_scope_thread>/barrier<thread_scope_thread> with memcpy_async is legal

According to the thread scope documentation:

> A thread scope specifies the kind of threads that can synchronize with each other using a primitive such as an atomic or a barrier.
>
> Each thread (CPU or GPU) is related to itself by the thread thread scope, specified with thread_scope_thread.

[This example](https://nvidia.github.io/libcudacxx/extended_api/synchronization_primitives/pipeline_consumer_wait_prior.html#example), however, suggests that multiple threads of execution - the current thread and the threads performing the `memcpy_async` - can be synchronized using the synchronization primitives with `thread_scope_thread`.

A fix could be to change the documentation to:

> The thread thread scope relates each thread (CPU or GPU) and its `???` threads to itself.

where `???` would be a name for the set of threads that includes the ones used by the `memcpy_async`s.