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

Clarify how the addend is used in _HI relocation types like
R_AMDGPU_ABS32_HI based on the current behaviour of the Mesa and AMDPAL
ELF loaders.

This affects Mesa and AMDPAL because they use REL relocation records, so
the addend for these types is the 32-bit literal value from the
instruction being relocated. AMDHSA is not affected because it uses RELA
relocation records which have a 64-bit addend.