[AMDGPU][GFX7][DOC][NFC] Update assembler syntax description

Summary of changes:
- Updated MUBUF lds syntax (see https://reviews.llvm.org/D124485).
- Enabled literals with src0 of v_madak_f32, v_madmk_f32 (see https://reviews.llvm.org/D111067).
- Corrected LGKM_CNT description.
- Minor bug fixing.