update update_geom_defaults() docs (#6158)

Author: teunbrand

R/geom-defaults.R

@@ -11,7 +11,17 @@
 #' @keywords internal
 #' @note
 #' Please note that geom defaults can be set *en masse* via the `theme(geom)`
-#' argument.
+#' argument. The guidelines for when to use which function are as follows:
+#'
+#' * If you want to change defaults for all geoms in all plots, use
+#'   `theme_update(geom = element_geom(...))`.
+#' * If you want to change defaults for all geoms in a single plot, use
+#'   `+ theme(geom = element_geom(...))`.
+#' * If you want to change defaults for one geom in all plots, use
+#'   `update_geom_defaults()`.
+#' * If you want to change settings for one geom in a single plot, use fixed
+#'   aesthetic parameters in a layer, like so: `geom_point(colour = "red")`.
+#'
 #' @export
 #' @examples
 #'