[AMDGPU] Update AMDGOUUsage.rst descriptions


- Improve description of XNACK ELF flag.
- Rename all uses of wave to wavefront to be consistent.

Differential Revision: https://reviews.llvm.org/D43983


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326989 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed