Document we should probably use React.memo under our Context Provider

It is not immediately obvious that when the component that declares the context provider re-renders, all its descendants will also render, and not just the components that subscribe to the context.

This has been made explicit by @sophiebits on Twitter: https://twitter.com/sophiebits/status/1228942768543686656, and @markerikson on his blog: https://blog.isquaredsoftware.com/2020/05/blogged-answers-a-mostly-complete-guide-to-react-rendering-behavior/#context-updates-and-render-optimizations, but following the right people on twitter should not be a requirement to understand how React is supposed to be used.

While it's true that if one reasons about how React decides to render components (given one has a sufficient understanding of it) this behaviour can be inferred, I still think that an explicit recommendation would help a majority of React users.

Happy to try submitting a PR if needed :)

未指定特定檔案或測試。請在 react.dev 文件中找到 React Context Provider 指南，並檢視連結的 rendering 參考；當文件清楚說明子孫元件會重新 rendering，並陳述預期的 React.memo 建議時，即表示完成。