[AMDGPU] Update AMD GPU documentation

- AMDGPUUsage.rst: Correct AMD GPU DWARF address space table address
  sizes which are in bits and not bytes.

- clang/.../Options.td: Improve description of AMD GPU options.

- Re-generate ClangComamndLineReference.rst from clang/.../Options.td .

Differential Revision: https://reviews.llvm.org/D90364