Add more information about scaling horizontally aytomatically connects containers to the DBs

[mickael-plassard]

A customer came to support in ticket 12375700351032 asking if when you scale a production app horizontally, does the new containers will add data to the same DB or not.

We do not answer that question in our documentation here: https://doc.scalingo.com/platform/app/scaling

Adding a small part about the fact that when adding containers to an app, the new containers connect automatically to the same DB as the other ones already working in that application here could be beneficial for users.