[AMDGPU] Correct formatting typos in documentation

Summary:
- Correct missing space in some "note" and "TODO" directives in
  AMDGPUUsage.rst
- Correct warning for heading underline being too short in
  BitCodeFormat.rst

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80407