With pbr 2.0 and Sphinx 1.5, the setting for treat sphinx warnings as
errors is setting warning-is-error in build_sphinx section. Migrate the
setting from the old warnerrors one.
This involves a couple of changes to the docs to fix the now broken
build.
Change-Id: I9bee7f6f9c69a9ed7bb22325ad1a0a1316dde526