Added text to show how to add color to legends for all levels, even w…

…hen no data for some levels exists.

Fixes #5869

R/layer.R

@@ -58,7 +58,9 @@
 #' `NA`, the default, includes if any aesthetics are mapped.
 #' `FALSE` never includes, and `TRUE` always includes.
 #' It can also be a named logical vector to finely select the aesthetics to
-#' display.
+#' display. This option can be used to include color for all levels, even 
+#' when no data exists, use `TRUE`. If 'FALSE', all levels are shown in legend, 
+#' but omitting colors for unobserved levels.
 #' @param inherit.aes If `FALSE`, overrides the default aesthetics,
 #' rather than combining with them. This is most useful for helper functions
 #' that define both data and aesthetics and shouldn't inherit behaviour from