Add cxx_flags to docs

[martinloretzzz]

Fixes #454 (Opened the issue half an hour ago, found that it's already implemented, but missing in the docs).

When I tried to add cxx_flags with the underscore to the build.toml, I got the following error: Unknown field cxx_flags. Expected one of cxx-flags, depends, include, src, that means likely the cuda_flags and sycl_flags should also be cuda-flags sycl-flags, should I also update this in the docs?

[danieldk]

Thanks, great catch! Is should indeed also be cuda-flags and sycl-flags, would be great if you could add them to this PR.