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: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Acked-by: NAleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-8-peter.maydell@linaro.org
Message-id: 20190228145624.24885-8-peter.maydell@linaro.org

docs/conf.py

@@ -77,6 +77,9 @@ pygments_style = 'sphinx'
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False
 
+# Sphinx defaults to warning about use of :option: for options not defined
+# with "option::" in the document being processed. Turn that off.
+suppress_warnings = ["ref.option"]
 
 # -- Options for HTML output ----------------------------------------------