message: [docs] improve upgrading instructions
diff: @@ -616,6 +616,18 @@ Upgrading should be as straightforward as running::
  superset db upgrade
  superset init
  +We recommend to follow standard best practices when upgrading Superset, such
  +as taking a database backup prior to the upgrade, upgrading a staging
  +environment prior to upgrading production, and upgrading production while less
  +users are active on the platform.
  +
  +.. note ::
  + Some upgrades may contain backward-incompatible changes, or require
  + scheduling downtime, when that is the case, contributors attach notes in
  + ``UPDATING.md`` in the repository. It's recommended to review this
  + file prior to running an upgrade.
  +
  +
  Celery Tasks
  ------------
  On large analytic databases, it's common to run background jobs, reports
  