Update R handles in-place (#6903)

* update R handles in-place #fixes 6896

* update test to expect non-null handle

* remove unused variable

* fix failing tests

* solve linter complains

R-package/man/xgb.unserialize.Rd

@@ -4,10 +4,17 @@
 \alias{xgb.unserialize}
 \title{Load the instance back from \code{\link{xgb.serialize}}}
 \usage{
-xgb.unserialize(buffer)
+xgb.unserialize(buffer, handle = NULL)
 }
 \arguments{
 \item{buffer}{the buffer containing booster instance saved by \code{\link{xgb.serialize}}}
+
+\item{handle}{An \code{xgb.Booster.handle} object which will be overwritten with
+the new deserialized object. Must be a null handle (e.g. when loading the model through
+`readRDS`). If not provided, a new handle will be created.}
+}
+\value{
+An \code{xgb.Booster.handle} object.
 }
 \description{
 Load the instance back from \code{\link{xgb.serialize}}