Update docs for how to upgrade sourcegraph.com

Created by: beyang

Changes merged to master in deploy-sourcegraph should land in deploy-sourcegraph-dot-com very quickly. This is in response to the alerts caused by Uwe’s change not getting deployed to sourcegraph.com for 1 month (and then we discovered issues).

Additional feedback from Thorsten, Eric, Stephen: https://sourcegraph.slack.com/archives/CMBA8F926/p1585908278034100