[media] V4L: Improve the selection API documentation

Make the VIDIOC_G/S_SELECTION ioctls documentation more consistent
with the rest of media Docbook, use capital letters where necessary
and correct few minor errors.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>