Clarify TiKV large-row limit in transaction restraints (#21830)#21833

ti-chi-bot wants to merge 3 commits into

pingcap:release-8.1pingcap/docs-cn:release-8.1from

ti-chi-bot:cherry-pick-21830-to-release-8.1ti-chi-bot/docs-cn:cherry-pick-21830-to-release-8.1Copy head branch name to clipboard