Document the Redundancy : Degraded status

[mcimbaro]

After a major PG upgrade, the database enter a new state Redundancy : Degraded.

It means that the secondary instance is reconstructing from the master instance data (Step 7 of Major-Upgrade Process > For Business Plans https://doc.scalingo.com/databases/postgresql/upgrading#process-1). We should document the meaning of this state, and the actions that are forbidden while the DB is in the current task. From my understanding, the only unallowed action in this state is to start a new DB (major ?) upgrade.

[Frzk]

I'm more than OK to specify the state in the documentation.
But shouldn't we detect the state and prevent the user to launch the (major-)upgrade when the database is in this state?