[AMDGPU] AMDGPUUsage clarify address space information and other typo and formatting...

[AMDGPU] AMDGPUUsage clarify address space information and other typo and formatting fixes

Summary:
- Clarify AMDGPU address spaces.
- Correct path to AMDGPU backend since now in the mono-repo.
- Fix numerous text style and typo issues.
- Correct reStructure text formatting warnings.
- Made reStructure directive usage more consistent.
- Add references for gfx10 ISA specification.

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71392