[R-package] various fixes for R CMD check (#1328)

* [R] fix xgb.create.features

* [R] fixes for R CMD check

R-package/man/print.xgb.cv.Rd

@@ -4,7 +4,7 @@
 \alias{print.xgb.cv.synchronous}
 \title{Print xgb.cv result}
 \usage{
-print.xgb.cv.synchronous(x, verbose = FALSE, ...)
+\method{print}{xgb.cv.synchronous}(x, verbose = FALSE, ...)
 }
 \arguments{
 \item{x}{an \code{xgb.cv.synchronous} object}