add documentation notes

R-package/R/xgb.DMatrix.save.R

@@ -1,10 +1,23 @@
-# save model or DMatrix to file
-xgb.DMatrix.save <- function(handle, fname) {
+#' Save xgb.DMatrix object to binary file
+#' 
+#' Save xgb.DMatrix object to binary file
+#' 
+#' @param model the model object.
+#' @param fname the name of the binary file.
+#' 
+#' @examples
+#' data(iris)
+#' iris[,5] <- as.numeric(iris[,5])
+#' dtrain <- xgb.DMatrix(as.matrix(iris[,1:4]), label=iris[,5])
+#' xgb.DMatrix.save(dtrain, 'iris.xgb.DMatrix')
+#' dtrain <- xgb.DMatrix('iris.xgb.DMatrix')
+#' 
+xgb.DMatrix.save <- function(DMatrix, fname) {
   if (typeof(fname) != "character") {
     stop("xgb.save: fname must be character")
   }
-  if (class(handle) == "xgb.DMatrix") {
-    .Call("XGDMatrixSaveBinary_R", handle, fname, as.integer(FALSE), 
+  if (class(DMatrix) == "xgb.DMatrix") {
+    .Call("XGDMatrixSaveBinary_R", DMatrix, fname, as.integer(FALSE), 
           PACKAGE = "xgboost")
     return(TRUE)
   }