[AMDGPU][NFC] Update DW_OP_LLVM_overlay documentation

Update DWARF Extensions For Heterogeneous Debugging proposal for the
DW_OP_LLVM_overlay operation:

1. Add an example.
2. Correct typo in definition of rbss.
3. Correct definition to specify both operands of the
   DW_OP_bit_piece operations.

Reviewed By: zoran.zaric

Differential Revision: https://reviews.llvm.org/D135394