[llvm] [AMDGPU][docs] define cluster scope and multicast loads (gfx1250) (PR #225310)

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh commented:

mostly nitpicking the wording which is a bit unusual in some places but otherwise this looks good

https://github.com/llvm/llvm-project/pull/225310