pd/tls: add upgrade tls description (#2710)#2713

ti-chi-bot[bot] merged 5 commits intopingcap:release-1.6pingcap/docs-tidb-operator:release-1.6from

ti-chi-bot:cherry-pick-2710-to-release-1.6ti-chi-bot/docs-tidb-operator:cherry-pick-2710-to-release-1.6Copy head branch name to clipboard