Add docs about cluster_id row in mysql.tidb (#19831)#21439

ti-chi-bot wants to merge 1 commit intopingcap:release-8.5pingcap/docs-cn:release-8.5from

ti-chi-bot:cherry-pick-19831-to-release-8.5ti-chi-bot/docs-cn:cherry-pick-19831-to-release-8.5Copy head branch name to clipboard