[Clang] Update the help message for --offload-arch

Summary:
This works for `OpenMP` and supports the `native` option. It should be
better documented.

Author: jhuber6

clang/include/clang/Driver/Options.td

@@ -940,10 +940,10 @@ def cuda_include_ptx_EQ : Joined<["--"], "cuda-include-ptx=">, Flags<[NoXarchOpt
 def no_cuda_include_ptx_EQ : Joined<["--"], "no-cuda-include-ptx=">, Flags<[NoXarchOption]>,
   HelpText<"Do not include PTX for the following GPU architecture (e.g. sm_35) or 'all'. May be specified more than once.">;
 def offload_arch_EQ : Joined<["--"], "offload-arch=">, Flags<[NoXarchOption]>,
-  HelpText<"CUDA offloading device architecture (e.g. sm_35), or HIP offloading target ID in the form of a "
-           "device architecture followed by target ID features delimited by a colon. Each target ID feature "
-           "is a pre-defined string followed by a plus or minus sign (e.g. gfx908:xnack+:sramecc-).  May be "
-           "specified more than once.">;
+  HelpText<"Specify an offloading device architecture for CUDA, HIP, or OpenMP. (e.g. sm_35). "
+           "If 'native' is used the compiler will detect locally installed architectures. "
+           "For HIP offloading, the device architecture can be followed by target ID features "
+           "delimited by a colon (e.g. gfx908:xnack+:sramecc-). May be specified more than once.">;
 def cuda_gpu_arch_EQ : Joined<["--"], "cuda-gpu-arch=">, Flags<[NoXarchOption]>,
   Alias<offload_arch_EQ>;
 def cuda_feature_EQ : Joined<["--"], "cuda-feature=">, HelpText<"Manually specify the CUDA feature to use">;