docs: clarify `Internationalization (i18n) Routing` (#63853)
## Description
Updated [Internationalization (i18n)
Routing](https://nextjs.org/docs/pages/building-your-application/routing/internationalization#leveraging-the-next_locale-cookie)
to clarify that `NEXT_LOCALE` does not override the `accept-language`
header value; rather, it prioritizes the `NEXT_LOCALE` cookie value over
`accept-language`.
## How has this been tested?
Ran `pnpm prettier-fix`
