[doc] Clarify the effect of enable_categorical (#9877)

R-package/R/xgb.DMatrix.R

@@ -27,14 +27,24 @@
 #' @param label_lower_bound Lower bound for survival training.
 #' @param label_upper_bound Upper bound for survival training.
 #' @param feature_weights Set feature weights for column sampling.
+#' @param enable_categorical Experimental support of specializing for categorical features.
+#'
+#'                           If passing 'TRUE' and 'data' is a data frame,
+#'                           columns of categorical types will automatically
+#'                           be set to be of categorical type (feature_type='c') in the resulting DMatrix.
+#'
+#'                           If passing 'FALSE' and 'data' is a data frame with categorical columns,
+#'                           it will result in an error being thrown.
+#'
+#'                           If 'data' is not a data frame, this argument is ignored.
+#'
+#'                           JSON/UBJSON serialization format is required for this.
 #'
 #' @details
 #' Note that DMatrix objects are not serializable through R functions such as \code{saveRDS} or \code{save}.
 #' If a DMatrix gets serialized and then de-serialized (for example, when saving data in an R session or caching
 #' chunks in an Rmd file), the resulting object will not be usable anymore and will need to be reconstructed
 #' from the original source of data.
-#' @param enable_categorical Experimental support of specializing for
-#'        categorical features. JSON/UBJSON serialization format is required.
 #'
 #' @examples
 #' data(agaricus.train, package='xgboost')