docs: watch_queue: fix some warnings

Fix those warnings:

    Documentation/watch_queue.rst:108: WARNING: Inline literal start-string without end-string.
    Documentation/watch_queue.rst:108: WARNING: Inline emphasis start-string without end-string.
    Documentation/watch_queue.rst:108: WARNING: Inline emphasis start-string without end-string.
    Documentation/watch_queue.rst:108: WARNING: Inline emphasis start-string without end-string.
    Documentation/watch_queue.rst:185: WARNING: Inline literal start-string without end-string.
    Documentation/watch_queue.rst:185: WARNING: Inline emphasis start-string without end-string.
    Documentation/watch_queue.rst:184: WARNING: Inline emphasis start-string without end-string.

The problem here is that the ``notation`` doesn't accept
multi lines. So, replace it to a code block using:

	::

		notation

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/42706310c09a6b4588a1a41078207246ad1238fa.1599660067.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>