docs/sphinx/kerneldoc.py: Honour --enable-werror

docs/sphinx/kerneldoc.py: Honour --enable-werror

Currently, the kerneldoc Sphinx plugin doesn't honour the
--enable-werror configure option, so its warnings are never fatal.
This is because although we do pass sphinx-build the -W switch, the
warnings from kerneldoc are produced by the scripts/kernel-doc script
directly and don't go through Sphinx's "emit a warning" function.

When --enable-werror is in effect, pass sphinx-build an extra
argument -Dkerneldoc_werror=1.  The kerneldoc plugin can then use
this to determine whether it should be passing the kernel-doc script
-Werror.

We do this because there is no documented mechanism for
a Sphinx plugin to determine whether sphinx-build was
passed -W or not; if one is provided then we can switch to
that at a later date:
https://github.com/sphinx-doc/sphinx/issues/11239

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230314114431.1096972-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>