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Documentation on how to call the subdev ops is currently in the middle of
synchronous and asynchronous registration. Move it to a dedicated
subsection after the registration methods.

Also move the final paragraph "The advantage of using v4l2_subdev..." to
the beginning of the new section as it has an introductory content.

[hverkuil: correct accidental deletion of the last part of a sentence]
Suggested-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>