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

从链接的 React Component 文档 react-component.html#fragments 部分开始，然后阅读链接的 React issue 及其关于 iterator 和 iterable 行为的 Seq 讨论。更新文档，说明 Component.render 可能返回 iterator，解释仅限 production 的限制，并建议使用 iterable，以便在两种环境中获得一致的行为。