[llvm] [RISCV][doc] Add note to RISCVUsage about supported atomics ABIs (PR #103879)

[Paul Kirth via llvm-commits]

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

LGTM on the new text. I left another potential suggestion but I’m also fine with the current wording. 

https://github.com/llvm/llvm-project/pull/103879