Mention that `Component.render` could return iterator

According to the discussion here https://github.com/facebook/react/issues/11502 it is legal to return iterators from Component.render. But it is not mentioned in the document.

Moreover -- it would work only in production mode, not in development. It should me mentioned that to make it work in both environments -- you should use iterable, rather than iterator (see this Seq approach from the Reach issue).

I suggest to place it around https://reactjs.org/docs/react-component.html#fragments

リンク先の React Component ドキュメントの react-component.html#fragments セクションから始め、次に iterator と iterable の挙動について、リンク先の React issue とその Seq discussion を読んでください。Component.render が iterator を返せることを記載するようドキュメントを更新し、production のみの制限について説明し、両方の環境で一貫した挙動になるよう iterable を推奨してください。