[AMDGPU][GFX90a][DOC][NFC] Update assembler syntax description

Summary of changes:
- Update MUBUF lds syntax (see https://reviews.llvm.org/D124485).
- Update SMEM syntax (see https://reviews.llvm.org/D127314).
- Enable src0=literal for v_madak*, v_madmk* (see https://reviews.llvm.org/D111067).
- Correct src0 operands of v_accvgpr_write_b32.
- Correct description of s_getreg/s_setreg (add TBA/TMA).
- Remove SYSMSG_OP_HOST_TRAP_ACK message.
- Minor bug fixing and improvements.