[clang] [RFC][Docs][Clang][AMDGPU] Add AMDGPU builtins documentation (PR #181193)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I wonder if we could put doc strings in the builtins.td file itself. I'm not sure if we need this for *every* single one, but it's definitely unfortunate how a lot of these I can only figure out how they work through the tests and ISA documenation.

https://github.com/llvm/llvm-project/pull/181193