[llvm] [RISCV] Add documentation in the LangRef on GHC CC (PR #72762)

[Alex Bradbury via llvm-commits]

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

If someone else had a chance to re-check the RISC-V specific limitations that would be great, but LGTM from what I can see.

https://github.com/llvm/llvm-project/pull/72762