[media] dvb: document dvb_frontend_sleep_until()

This function is used mainly at the DVB core, in order to provide
emulation for a legacy ioctl. The only current exception is
the stv0299 driver, with takes more than 8ms to switch voltage,
breaking the emulation for FE_DISHNETWORK_SEND_LEGACY_CMD.

Document that.

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