[llvm] Update documentation for buffer fat pointers (PR #92034)

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

One thing I noticed while writing this: should the AMDGPU metadata streamer be updated so that `ptr addrspace(8)` inputs to kernels get the type "image" and not "global buffer"?

https://github.com/llvm/llvm-project/pull/92034