[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/man/xgb.load.Rd

@@ -7,10 +7,22 @@
 xgb.load(modelfile)
 }
 \arguments{
-\item{modelfile}{the name of the binary file.}
+\item{modelfile}{the name of the binary input file.}
+}
+\value{
+An object of \code{xgb.Booster} class.
 }
 \description{
-Load xgboost model from the binary model file
+Load xgboost model from the binary model file.
+}
+\details{
+The input file is expected to contain a model saved in an xgboost-internal binary format
+using either \code{\link{xgb.save}} or \code{\link{cb.save.model}} in R, or using some 
+appropriate methods from other xgboost interfaces. E.g., a model trained in Python and 
+saved from there in xgboost format, could be loaded from R.
+
+Note: a model saved as an R-object, has to be loaded using corresponding R-methods,
+not \code{xgb.load}.
 }
 \examples{
 data(agaricus.train, package='xgboost')
@@ -23,4 +35,7 @@ xgb.save(bst, 'xgb.model')
 bst <- xgb.load('xgb.model')
 pred <- predict(bst, test$data)
 }
+\seealso{
+\code{\link{xgb.save}}, \code{\link{xgb.Booster.complete}}.
+}