Add example to map_dfr()/map_dfc() (#1140)

Example to demonstrate an edge case, where map_dfr() works, but can not be replaced by list_rbind(). Fixes #1074

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Co-authored-by: Maike <maike2011@gmx-topmail.de>
Co-authored-by: Hadley Wickham <h.wickham@gmail.com>

R/superseded-map-df.R

@@ -41,6 +41,23 @@
 #'   map(\(mod) as.data.frame(t(as.matrix(coef(mod))))) |>
 #'   list_rbind()
 #'
+#' # for certain pathological inputs `map_dfr()` and `map_dfc()` actually
+#' # both combine the list by column
+#' df <- data.frame(
+#'   x = c(" 13", "  15 "),
+#'   y = c("  34",  " 67 ")
+#' )
+#'
+#' # Was:
+#' map_dfr(df, trimws)
+#' map_dfc(df, trimws)
+#'
+#' # But list_rbind()/list_cbind() fail because they require data frame inputs
+#' try(map(df, trimws) |> list_rbind())
+#'
+#' # Instead, use modify() to apply a function to each column of a data frame
+#' modify(df, trimws)
+#'
 #' # map2 ---------------------------------------------
 #'
 #' ex_fun <- function(arg1, arg2){