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

先從連結的 React Component 文件中 react-component.html#fragments 區段開始，接著閱讀連結的 React issue 及其關於 iterator 與 iterable 行為的 Seq 討論。更新文件，說明 Component.render 可能會回傳 iterator，解釋僅限於 production 的限制，並建議使用 iterable，以便在兩種環境中維持一致的行為。