Add labeller example for optional argument. Fixes #5916 (#6038)

Author: sierrajohnson

R/labeller.R

@@ -83,6 +83,9 @@
 #' # Interpreting the labels as plotmath expressions
 #' p + facet_grid(. ~ cyl2)
 #' p + facet_grid(. ~ cyl2, labeller = label_parsed)
+#'
+#' # Include optional argument in label function
+#' p + facet_grid(. ~ cyl, labeller = function(x) label_both(x, sep = "="))
 #' }
 #' @name labellers
 NULL