Treat warnings in Sphinx as errors. The reasons for doing this are...

- When CMake builds the documentation with sphinx-build it treats
  warnings as errors. We should be consistent with what we do in
  CMake.
- Having warnings treated as errors will hopefully encourage
  developers to write documentation correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213661 91177308-0d34-0410-b5e6-96231b3b80d8