patch 8.2.4240: error for using flatten() in Vim9 script is unclear

Problem:    Error for using flatten() in Vim9 script is unclear.
Solution:   Add a remark to use flattennew().

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4240,
 /**/
     4239,
 /**/