[AMDGPU] Clarify Code Object V2/V3 differences in AMDGPUUsage

Ensure Code Object V2 documentation is complete, but always contains a
warning and a link to the equivalent Code Object V3 documentation.

Explicitly indicate that any note records present in a code object that
are not documented must be considered deprecated and ignored.

Differential Revision: https://reviews.llvm.org/D59782

llvm-svn: 357176