Explain how to fix fill guide for points with filled shapes (#6229)

* Explain how to fix fill guide for points with filled shapes

* Refine filled point example in docs

Author: arcresu

R/geom-point.R

@@ -85,6 +85,13 @@
 #' ggplot(mtcars, aes(wt, mpg)) +
 #'   geom_point(shape = 21, colour = "black", fill = "white", size = 5, stroke = 5)
 #'
+#' # The default shape in legends is not filled, but you can override the shape
+#' # in the guide to reflect the fill in the legend
+#' ggplot(mtcars, aes(wt, mpg, fill = factor(carb), shape = factor(cyl))) +
+#'   geom_point(size = 5, stroke = 1) +
+#'   scale_shape_manual(values = 21:25) +
+#'   scale_fill_ordinal(guide = guide_legend(override.aes = list(shape = 21)))
+#'
 #' \donttest{
 #' # You can create interesting shapes by layering multiple points of
 #' # different sizes

man/geom_point.Rd

@@ -251,6 +251,8 @@ ggplot(sizes, aes(size, stroke, size = size, stroke = stroke)) +
   scale_size_identity()
 ```
 
+Because points are not typically filled, you may need to change some default settings when using these shapes and mapping `fill`. In particular, discrete `fill` guides will be drawn with an unfilled shape unless overridden (refer to `geom_point()` for an example of this).
+
 ## Text
 
 ### Font family