[media] DocBook: better document FE_SET_VOLTAGE ioctl

Use the proper format for FE_SET_VOLTAGE documentation and fix
the documentation. The description for the enum is not 100%,
and it is missing the voltage off value.

Also, it is better to keep the enum description together with
the ioctl, as both are used together.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>