[AMDGPU][GFX940][DOC][NFC] Update assembler syntax description

Summary of changes:
- Enable VOP3 variants of dot2c/dot4c/dot8c instructions (https://reviews.llvm.org/D138494).
- Enable abs and neg modifiers for v_cndmask_b32 (https://reviews.llvm.org/D135900).
- Enable omod modifiers for v_max3_f16, v_min3_f16, etc. (https://reviews.llvm.org/D139469).
- Correct v_mov_b32_sdwa (it does not support abs and neg input modifiers yet).
- Enable abs and neg modifiers for v_dot2c_f32_f16_dpp.
- Minor corrections and improvements.