Change tidb_memory_usage_alarm_ratio scope to instance (#5988)#5994

ti-chi-bot merged 2 commits into

pingcap:release-5.1pingcap/docs:release-5.1from

ti-chi-bot:cherry-pick-5988-to-release-5.1ti-chi-bot/docs:cherry-pick-5988-to-release-5.1Copy head branch name to clipboard