[media] Document the obscure dvb_frontend_reinitialise()

The dvb_frontend_reinitialise() function is a special case
used by just one frontend. Document it, for completeness.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-core/dvb_frontend.h

@@ -694,10 +694,23 @@ extern int dvb_unregister_frontend(struct dvb_frontend *fe);
 
 extern void dvb_frontend_detach(struct dvb_frontend *fe);
 
-extern void dvb_frontend_reinitialise(struct dvb_frontend *fe);
 extern int dvb_frontend_suspend(struct dvb_frontend *fe);
 extern int dvb_frontend_resume(struct dvb_frontend *fe);
 
+/**
+ * dvb_frontend_reinitialise() - forces a reinitialisation at the frontend
+ *
+ * @fe: pointer to the frontend struct
+ *
+ * Calls &dvb_frontend_ops.init() and &dvb_frontend_ops.tuner_ops.init(),
+ * and resets SEC tone and voltage (for Satellite systems).
+ *
+ * NOTE: Currently, this function is used only by one driver (budget-av).
+ * It seems to be due to address some special issue with that specific
+ * frontend.
+ */
+void dvb_frontend_reinitialise(struct dvb_frontend *fe);
+
 /**
  * dvb_frontend_sleep_until() - Sleep for the amount of time given by
  *                      add_usec parameter