[AMDGPU][GFX8][DOC][NFC] Update assembler syntax description

Summary of changes:
- Enable register tuples with 9, 10, 11 and 12 registers (https://reviews.llvm.org/D138205).
- Enable abs and neg modifiers for v_cndmask_b32 (https://reviews.llvm.org/D135900).
- Correct v_mov_b32_sdwa (it does not support abs and neg input modifiers yet).
- Enable tfe modifier for MUBUF loads (https://reviews.llvm.org/D137783).
- Minor corrections and improvements.