add more description for null_model (#1291)

Author: libbymckenna

R/nullmodel.R

@@ -127,9 +127,18 @@ predict.nullmodel <- function (object, new_data = NULL, type  = NULL, ...) {
 
 #' Null model
 #'
+#' Fit a single mean or largest class model. `null_model()` is the user-facing
+#' function that relies on the underlying computational function, `nullmodel()`.
+#'
 #' `null_model()` defines a simple, non-informative model. It doesn't have any
 #'  main arguments. This function can fit classification and regression models.
 #'
+#' `null_model()` emulates other model building functions, but returns the
+#' simplest model possible given a training set: a single mean for numeric
+#' outcomes and the most prevalent class for factor outcomes. When class
+#' probabilities are requested, the percentage of the training set samples with
+#' the most prevalent class is returned.
+#'
 #' @param mode A single character string for the type of model. The only
 #' possible values for this model are `"regression"` and `"classification"`.
 #' @param engine A single character string specifying what computational engine