v8.5.5: add doc for async-batch-get (#21164)#21271

ti-chi-bot[bot] merged 7 commits intopingcap:feature/preview-v8.5.5pingcap/docs-cn:feature/preview-v8.5.5from

ti-chi-bot:cherry-pick-21164-to-release-8.5ti-chi-bot/docs-cn:cherry-pick-21164-to-release-8.5Copy head branch name to clipboard