[AMDGPU][GFX10][DOC][NFC] Update assembler syntax description

Summary of changes:
- Update MUBUF lds syntax (see https://reviews.llvm.org/D124485).
- Add v_cvt_pkrtz_f16_f32_dpp, v_cvt_pkrtz_f16_f32_sdwa.
- Update SMEM syntax (see https://reviews.llvm.org/D127314).
- Enable op_sel for v_add_nc_u16, v_sub_nc_u16 (see https://reviews.llvm.org/D123594).
- Minor bug fixing and improvements.