Current GTK+ from CVS has the following comment:

/**
 * gtk_window_get_size:
 * @window: a #GtkWindow
 * @width: return location for width, or %NULL
 * @height: return location for height, or %NULL
 *
 * Obtains the current size of @window. If @window is not onscreen, it
 * returns the size GTK+ will suggest to the <link ...

For whatever reason gtk-doc thinks that the line starting with * returns
is describing the function's return value:

 WARNING: Parameter described in source code comment block but does not exist -
         Func: gtk_window_get_size Param: Returns.

Actually the gtk-doc version I'm using is current CVS (0.9) but bugzilla
doesn't offer this version.