tidb: update tidb_index_serial_scan_concurrency docs (#21938)#23268

ti-chi-bot[bot] merged 7 commits into

pingcap:release-6.5pingcap/docs:release-6.5from

ti-chi-bot:cherry-pick-21938-to-release-6.5ti-chi-bot/docs:cherry-pick-21938-to-release-6.5Copy head branch name to clipboard