Document breaking change in repository name configuration

[entangled90]

Because indices have been harmonized, the ES/OS repository name must be the same in all webapps or (if deployed as a single app, only one of the has to be configured)

Context:

If users are deploying the web apps in standalone mode, and they configure different snapshot repository names for each, then the backups will be taken based on which node serves the request.

• This can mean missing back ups on restore if a different node serves the request.
• This can mean more expensive back ups since incremental backups would not work as well. It's unclear what was previously the deployment configuration, so we will assume users defined the same name across all apps.
• We will document that it's important they configure either only one of them, or that all repository name configuration are the same, and we will document why.
• If users do not read our docs or misconfigure things, then they may have a bad UX, but the backups should still exist they might just be harder to restore.
  -It's unclear how we would move their snapshots from one repository to the other or something like this, but that's the same as if they changed the name after the initial deployment, so...I guess it's OK for now.