dumpling: clarify description about --csv-separator (#21126)#21131

ti-chi-bot[bot] merged 3 commits intopingcap:release-6.1pingcap/docs-cn:release-6.1from

ti-chi-bot:cherry-pick-21126-to-release-6.1ti-chi-bot/docs-cn:cherry-pick-21126-to-release-6.1Copy head branch name to clipboard