[backport] [R] Fix document for nthread. (#7263) (#7269)

R-package/man/xgb.DMatrix.Rd

@@ -4,7 +4,14 @@
 \alias{xgb.DMatrix}
 \title{Construct xgb.DMatrix object}
 \usage{
-xgb.DMatrix(data, info = list(), missing = NA, silent = FALSE, ...)
+xgb.DMatrix(
+  data,
+  info = list(),
+  missing = NA,
+  silent = FALSE,
+  nthread = NULL,
+  ...
+)
 }
 \arguments{
 \item{data}{a \code{matrix} object (either numeric or integer), a \code{dgCMatrix} object, or a character
@@ -18,6 +25,8 @@ It is useful when a 0 or some other extreme value represents missing values in d
 
 \item{silent}{whether to suppress printing an informational message after loading from a file.}
 
+\item{nthread}{Number of threads used for creating DMatrix.}
+
 \item{...}{the \code{info} data could be passed directly as parameters, without creating an \code{info} list.}
 }
 \description{