show.legend for factor data for all levels, even when not observed (#6037)

* Added text to show how to add color to legends for all levels, even when no data for some levels exists.

Fixes #5869

* Update R/layer.R

edits from Tuen

Co-authored-by: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>

* Includes edits from Teun to clarify language and ran `devtools::document()`

---------

Co-authored-by: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>

Author: agmurray

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. To include legend keys for all levels, even 
+#'   when no data exists, use `TRUE`.  If `NA`, all levels are shown in legend, 
+#'   but unobserved levels are omitted.
 #' @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