system-variables: add documents for error_count and max_connections (#10525)#10655

ti-chi-bot merged 4 commits into

pingcap:release-5.2pingcap/docs:release-5.2from

ti-chi-bot:cherry-pick-10525-to-release-5.2ti-chi-bot/docs:cherry-pick-10525-to-release-5.2Copy head branch name to clipboard