document new behaviour

Author: teunbrand

R/theme.R

@@ -151,8 +151,10 @@
 #'   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. Using this setting overrides any
-#'   aspect ratio set by the theme, coord or facets.
+#'   set the size of individual panels. Using this setting overrides the
+#'   aspect ratio set by the theme, coord or facets. An exception is made when
+#'   the plot has a single panel and exactly one of the width *or* height is
+#'   set, in which case an attempt is made to preserve the aspect ratio.
 #' @param panel.ontop option to place the panel (background, gridlines) over
 #'   the data layers (`logical`). Usually used with a transparent or blank
 #'   `panel.background`.