[R] various R code maintenance (#1964)

* [R] xgb.save must work when handle in nil but raw exists

* [R] print.xgb.Booster should still print other info when handle is nil

* [R] rename internal function xgb.Booster to xgb.Booster.handle to make its intent clear

* [R] rename xgb.Booster.check to xgb.Booster.complete and make it visible; more docs

* [R] storing evaluation_log should depend only on watchlist, not on verbose

* [R] reduce the excessive chattiness of unit tests

* [R] only disable some tests in windows when it's not 64-bit

* [R] clean-up xgb.DMatrix

* [R] test xgb.DMatrix loading from libsvm text file

* [R] store feature_names in xgb.Booster, use them from utility functions

* [R] remove non-functional co-occurence computation from xgb.importance

* [R] verbose=0 is enough without a callback

* [R] added forgotten xgb.Booster.complete.Rd; cran check fixes

* [R] update installation instructions

R-package/R/xgb.save.R

@@ -1,9 +1,22 @@
 #' Save xgboost model to binary file
 #' 
-#' Save xgboost model from xgboost or xgb.train
+#' Save xgboost model to a file in binary format.
 #' 
-#' @param model the model object.
-#' @param fname the name of the file to write.
+#' @param model model object of \code{xgb.Booster} class.
+#' @param fname name of the file to write.
+#' 
+#' @details 
+#' This methods allows to save a model in an xgboost-internal binary format which is universal 
+#' among the various xgboost interfaces. In R, the saved model file could be read-in later
+#' using either the \code{\link{xgb.load}} function or the \code{xgb_model} parameter 
+#' of \code{\link{xgb.train}}.
+#' 
+#' Note: a model can also be saved as an R-object (e.g., by using \code{\link[base]{readRDS}} 
+#' or \code{\link[base]{save}}). However, it would then only be compatible with R, and 
+#' corresponding R-methods would need to be used to load it.
+#' 
+#' @seealso 
+#' \code{\link{xgb.load}}, \code{\link{xgb.Booster.complete}}. 
 #' 
 #' @examples
 #' data(agaricus.train, package='xgboost')
@@ -22,6 +35,7 @@ xgb.save <- function(model, fname) {
   if (class(model) != "xgb.Booster")
     stop("the input must be xgb.Booster. Use xgb.DMatrix.save to save xgb.DMatrix object.")
   
+  model <- xgb.Booster.complete(model, saveraw = FALSE)
   .Call("XGBoosterSaveModel_R", model$handle, fname, PACKAGE = "xgboost")
   return(TRUE)
 }