media: v4l uAPI: add descriptions for arguments to all ioctls

Several ioctls are missing descriptions for the third argument
of the ioctl() command. They should have a description, as
otherwise the output won't be ok, and will sound like something
is missing.

So, add them.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>