clarify when dpi applies (#5842)

Author: teunbrand

R/save.R

@@ -37,7 +37,8 @@
 #' @param units One of the following units in which the `width` and `height`
 #'   arguments are expressed: `"in"`, `"cm"`, `"mm"` or `"px"`.
 #' @param dpi Plot resolution. Also accepts a string input: "retina" (320),
-#'   "print" (300), or "screen" (72). Applies only to raster output types.
+#'   "print" (300), or "screen" (72). Only applies when converting pixel units,
+#'   as is typical for raster output types.
 #' @param limitsize When `TRUE` (the default), `ggsave()` will not
 #'   save images larger than 50x50 inches, to prevent the common error of
 #'   specifying dimensions in pixels.