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

Nadie ha tomado este issue todavía.

## 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.

Abre el artículo Keyboard: keydown y keyup y revisa la sección Default actions, especialmente el ejemplo del filtro de números de teléfono y la explicación del input event en la introducción. Añade una nota breve que explique la limitación de IME y por qué input es preferible para hacer un seguimiento del texto insertado o validarlo; después, verifica la redacción con el ejemplo.

Escrito por el modelo de indexación a partir del texto del issue.