Update AMDGPUUsage.rst documentation:

    1. Correct description of the kernel initial state for FLAT_SCRATCH_INIT.
    2. Add link to GFX9 architecture documentation.
    3. Update product names.
    4. Rename note record from NT_AMD_AMDGPU_METADATA to NT_AMD_AMDGPU_HSA_METADATA and move description to the AMDHSA coding convention section.
    5. Minor typo corrections.

Differential Revision: https://reviews.llvm.org/D36549

llvm-svn: 310954