Add note about dev/build behavior to error boundaries docs

I'm using create-react-app and when I use an ErrorBoundary with componentdidcatch and run "npm start," my custom error message briefly flashes as intended, but then the regular developer error page shows after a second or so. There is no mention of this behavior on the docs page, so I spent a lot of time trying to figure out what I did wrong before I tried running the build version and saw it worked as expected (no default error page shown).

找到 Error Boundaries 文件頁面，並使用 "npm start" 和 build 版本透過 create-react-app 重現該行為。新增註記，說明在開發環境中開發者錯誤頁面可能會出現在自訂訊息之後，而 build 版本的行為符合預期；確認文件與兩種模式都一致。