Add best practice GPU runtime evironment variables for running on Frontier and other HPC target to Manual

[PDoakORNL]

Sufficient explanation to allow extrapolation to other systems would be helpful as well.

[prckent]

I added a mention of LIBOMPTARGET_AMDGPU_MAX_ASYNC_COPY_BYTES=0 in #5399 . Currently, no other environment variables should be modified from the OLCF defaults.

@Hyeondeok-Shin Could you please add a sample Aurora job file in the docs with any environment variables that need setting?

AFAIK, we don't need any special settings for NVIDIA with recent CUDA versions used in combination with recent mainline LLVM releases.