fix comments (#298)

qbft/controller.go

@@ -47,11 +47,12 @@ func (c *Controller) StartNewInstance(height Height, value []byte) error {
 		return errors.Wrap(err, "value invalid")
 	}
 
+	// can't use <= because of height == 0 case
 	if height < c.Height {
 		return errors.New("attempting to start an instance with a past height")
 	}
 
-	// only if current height's instance exists (and decided since passed can start instance) bump
+	// covers height == 0 case
 	if c.StoredInstances.FindInstance(height) != nil {
 		return errors.New("instance already running")
 	}