media: docs: v4l2-subdev: move calling ops to a subsection

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>