Document splicing argument in theme() (#5790)

* Update theme.R

* Update R/theme.R

Co-authored-by: Teun van den Brand <[email protected]>

* document()

---------

Co-authored-by: Teun van den Brand <[email protected]>

Author: olivroy

R/theme.R

@@ -194,7 +194,7 @@
 #'   switched (`unit`)
 #'
 #' @param ... additional element specifications not part of base ggplot2. In general,
-#'   these should also be defined in the `element tree` argument.
+#'   these should also be defined in the `element tree` argument. [Splicing][rlang::splice] a list is also supported.
 #' @param complete set this to `TRUE` if this is a complete theme, such as
 #'   the one returned by [theme_grey()]. Complete themes behave
 #'   differently when added to a ggplot object. Also, when setting