tiflash: add warning to upgrade from pre-rc version (#2319)#2323

sre-bot merged 2 commits into

pingcap:release-4.0pingcap/docs:release-4.0from

sre-bot:release-4.0-3fc2a9e9f88fsre-bot/docs:release-4.0-3fc2a9e9f88fCopy head branch name to clipboard