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().
diff --git a/src/version.c b/src/version.c
index 3d37eec..fff4e6d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4240,
+/**/
     4239,
 /**/
     4238,