refine vignette

R-package/R/xgb.save.R

@@ -21,6 +21,7 @@ xgb.save <- function(model, fname) {
     .Call("XGBoosterSaveModel_R", model, fname, PACKAGE = "xgboost")
     return(TRUE)
   }
-  stop("xgb.save: the input must be either xgb.DMatrix or xgb.Booster")
+  stop("xgb.save: the input must be xgb.Booster. Use xgb.DMatrix.save to save
+       xgb.DMatrix object.")
   return(FALSE)
 }