improve the phrasing for using "no keys" on https://reactjs.org/docs/lists-and-keys.html#keys

https://reactjs.org/docs/lists-and-keys.html#keys currently has a section that causes confusion about React's behaviour when omitting key attributes, currently reading:

"We don’t recommend using indexes for keys if the order of items may change. This can negatively impact performance and may cause issues with component state. Check out Robin Pokorny’s article for an in-depth explanation on the negative impacts of using an index as a key. If you choose not to assign an explicit key to list items then React will default to using indexes as keys."

The emphasized sentence has led to folks thinking that React knows what a specific map()'s iteration index is, and that it'll use that, rather than that the index talked about is the child index of the element in the vdom. To remove that confusion and make it explicit that React is using an internal index, I would recommend changing the phrasing here to:

"We don’t recommend using indexes for keys if the order of items may change. This can negatively impact performance and may cause issues with component state. Check out Robin Pokorny’s article for an in-depth explanation on the negative impacts of using an index as a key. If you choose not to assign an explicit key to list items then React will default to using element index in the virtual DOM tree as keys."

It might be even more high value to also add one or more sentences to explain that this means that the behaviour for consecutive maps that don't use keys vs. incorrectly use iteration indices as keys will lead to different behaviour, which folks right now mistakenly assume would behave the same.

E.g.

render() {
  const { text, additional } = this.state;
  const content = text.map(t => <p>{t}</p>);
  const addenda = additional.map(t => <p className="add">{t}</p>);
  return <section>{content}{addenda}</section>;
}

will work, but:

render() {
  const { text, additional } = this.state;
  const content = text.map((t,i) => <p>{t}</p>);
  const addenda = additional.map((t,i) => <p className="add" key={i}>{t}</p>);
  return <section>{content}{addenda}</section>;
}

will not, and if you are familiar enough with React already to not need the docs, you know why, but if you're not and you read the docs, it is entirely understandable if you come away convinced that this should work after reading the current text.

https://reactjs.org/docs/lists-and-keys.html#keys の Keys セクションから始め、提供されている例と合わせて、Keys を省略した場合についての表現を確認してください。React は virtual DOM tree 内の要素のインデックスを使用することを明確にし、連続した map の例も追加すべきか判断してください。ドキュメントでは、両方の挙動を正確に区別する必要があります。