[llvm] [AMDGPU] Clarify description of _HI relocation types (PR #73663)

[Jay Foad via llvm-commits]

jayfoad wrote:

Here's the relevant part of the Mesa and AMDPAL ELF loaders:

Mesa: https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/amd/common/ac_rtld.c?ref_type=heads#L638
AMDPAL: https://github.com/GPUOpen-Drivers/pal/blob/6bdc69412b3c3936bcc6e23edf9e338d66b69d5f/src/core/hw/gfxip/pipeline.cpp#L1080

https://github.com/llvm/llvm-project/pull/73663