[llvm] [Doc][AMDGPU] Add barrier execution & memory model (PR #170447)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

Thanks. Most of my remaining comments are more of an editorial nature. The one comment I would like to see addressed is the one about linking waves and threads.

https://github.com/llvm/llvm-project/pull/170447