Keyboard events: keydown filtering example can't block IME-composed input (Korean/Japanese/Chinese)

まだ誰も着手していません。

## Article

[Keyboard: keydown and keyup](https://javascript.info/keyboard-events) — the "Default actions" section, phone-number filter example:

```html

```

## Problem

The example (and the surrounding text) implies that returning `false` from `keydown` reliably prevents unwanted characters from being entered. That holds for direct (Latin) keyboard input, but not for languages typed through an IME (Input Method Editor), such as Korean, Japanese and Chinese.

With an IME active:

- `keydown` fires with `event.key == "Process"` (and `keyCode` 229), not the actual character.
- More importantly, calling `preventDefault()` / returning `false` on that `keydown` does **not** cancel the text insertion, because the character is produced by the IME composition, not by the key's default action.

As a result, Korean/Japanese/Chinese characters pass straight through the filter and appear in the field. This was reported by a Korean reader of the Korean translation (ko.javascript.info), and is easy to reproduce by switching to a Korean IME and typing into the demo.

## Suggestion

Add a short note after the example, along these lines:

> Please note: this filter only works for direct keyboard input. Text composed via an IME (used for languages such as Korean, Japanese or Chinese) is inserted by the input method, not by the key's default action, so it can't be blocked in `keydown`. That's one more reason why, for tracking or validating what actually gets into the field, the `input` event is a better choice.

This ties in nicely with the introduction of the article, which already recommends the `input` event for tracking input from any source.

Happy to send a PR if the note sounds good.

Keyboard: keydown and keyup の記事を開き、Default actions セクションを確認してください。特に、電話番号フィルターの例と、導入部で説明されている input event について確認します。IME の制限と、挿入されたテキストの追跡や検証には input が適している理由を説明する短い注記を追加し、その後、例に照らして文言を確認してください。

索引モデルが issue の本文から書いたものです。