[AMDGPU][MC][NFC][DOC] Updated AMD GPU assembler syntax description.

Summary of changes:
- added description of MTBUF instructions and format modifier;
- described limitations of f16 inline constants when used with integer operands;
- updated description of gfx9+ flat global addressing modes;
- v_accvgpr_write_b32 src0 corrections (gfx908);
- minor bugfixing and improvements.