[NFC][AMDGPU] AMDGPU code object V4 ABI documentation

- Documantation for AMDGPU code object V4.
- Documentation clarification for code object V2 and V3.
- Documentation for the clang-offload-bundler.
- Numerous other documentation clarifications.

Change-Id: I338b327cc9e75da6c987b7e081b496402a5a020e

Differential Revision: https://reviews.llvm.org/D92434