[AMDGPU] DWARF proposal review feedback

- Rename DW_OP_LLVM_offset_constu to DW_OP_LLVM_offset_uconst to
  matches DW_OP_plus_uconst.
- Correct DW_OP_LLVM_call_ref to be DW_OP_call_ref.
- Move proposed changes to a separate section to clarify that the
  introduction section is not part of the changes.
- Fix formatting typos and add missing reference.
- Clarify why DW_OP_LLVM_offset et al do not wrap on overflow.
- Correct syntax of augmentation string.

Differential Revision: https://reviews.llvm.org/D70523