doc: note v15.0.0 changed default --unhandled-rejections=throw

PR-URL: #36361
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/cli.md

@@ -1059,8 +1059,13 @@ Track heap object allocations for heap snapshots.
 ### `--unhandled-rejections=mode`
 <!-- YAML
 added:
- - v12.0.0
- - v10.17.0
+  - v12.0.0
+  - v10.17.0
+changes:
+  - version: v15.0.0
+    pr-url: https://github.com/nodejs/node/pull/33021
+    description: Changed default mode to `throw`. Previously, a warning was
+                 emitted.
 -->
 
 Using this flag allows to change what should happen when an unhandled rejection