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

[Matt Arsenault via llvm-commits]

arsenm 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"?

I'm not sure what "image" is supposed to mean there, but an arbitrary resource descriptor isn't necessarily an image 

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