docs/conf.py: Disable option warnings

sphinx-build complains about using :option: to mark up option
flags that it doesn't know about (because they were not defined
using the "option::" directive):
docs/pr-manager.rst:68: WARNING: unknown option: -d

Suppress these warnings. This way we get the semantic markup
of the option flag but no cross-referencing hyperlink.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-8-peter.maydell@linaro.org
Message-id: 20190228145624.24885-8-peter.maydell@linaro.org