[media] docs-rst: media: better document refcount in struct dvb_frontend

The refcount field was added to the struct but it was not
properly documented.

Document it.

[mchehab@s-opensource.com: fix a merge conflict]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-core/dvb_frontend.h

@@ -643,7 +643,8 @@ struct dtv_frontend_properties {
 /**
  * struct dvb_frontend - Frontend structure to be used on drivers.
  *
- * @refcount:		pointer to struct kref
+ * @refcount:		refcount to keep track of struct dvb_frontend
+ *			references
  * @ops:		embedded struct dvb_frontend_ops
  * @dvb:		pointer to struct dvb_adapter
  * @demodulator_priv:	demod private data