[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>

include/uapi/linux/dvb/frontend.h

@@ -105,11 +105,13 @@ struct dvb_diseqc_slave_reply {
 };			/*  errorcode when no message was received  */
 
 
-typedef enum fe_sec_voltage {
+enum fe_sec_voltage {
 	SEC_VOLTAGE_13,
 	SEC_VOLTAGE_18,
 	SEC_VOLTAGE_OFF
-} fe_sec_voltage_t;
+};
+
+typedef enum fe_sec_voltage fe_sec_voltage_t;
 
 
 typedef enum fe_sec_tone_mode {