[SYCL][DOC] Add draft of sycl_ext_oneapi_async_memcpy#9439

dkhaldi wants to merge 2 commits intointel:syclintel/llvm:syclfrom

dkhaldi:sycl_ext_oneapi_async_memcpydkhaldi/llvm:sycl_ext_oneapi_async_memcpyCopy head branch name to clipboard