[doc] improve the doc for CUDA

1. Mentioned that CUDA support works best with trunk.
2. Simplified the example by removing its dependency on the CUDA samples.
3. Explain the --cuda-gpu-arch flag.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259307 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed