Add note in docs

Author: teunbrand

R/theme.R

@@ -151,7 +151,8 @@
 #'   from `line`
 #' @param panel.widths,panel.heights Sizes for panels (`units`). Can be a
 #'   single unit to set the total size for the panel area, or a unit vector to
-#'   set the size of individual panels.
+#'   set the size of individual panels. Using this setting overrides any
+#'   aspect ratio set by the theme, coord or facets.
 #' @param panel.ontop option to place the panel (background, gridlines) over
 #'   the data layers (`logical`). Usually used with a transparent or blank
 #'   `panel.background`.