dumpling: clarify description about --csv-separator (#21126)#21129

ti-chi-bot wants to merge 1 commit intopingcap:release-6.5pingcap/docs-cn:release-6.5from

ti-chi-bot:cherry-pick-21126-to-release-6.5ti-chi-bot/docs-cn:cherry-pick-21126-to-release-6.5Copy head branch name to clipboard