[llvm] [AMDGPU] Document that only naturally aligned atomics of up to 64 bits are supported by the AMDGPU backend (PR #200167)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

I'm sure this note will still be here after the support is implemented 

https://github.com/llvm/llvm-project/pull/200167