AMDGPU: Document address space mapping

Summary:
Address space mapping is described in lib/Target/AMDGPU/AMDGPU.h in
Doxygen comments. This patch adds the description to user guide for
AMDGPU back-end.

Patch By: Vedran Miletić

Reviewers: tstellarAMD, arsenm

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17046

llvm-svn: 265500