Geo-redundancy state cannot be changed once the server is created ([https://docs.microsoft.com/en-us/azure/mariadb/concepts-backup#backup-redundancy-options](link). This TF provider gives a somewhat generic error: 

`Error waiting for creation of MariaDB Server "svr-name" (Resource Group "rg-name"): Code="UnsupportedUpdate" Message="Cannot update GeoRedundantBackup for server"`

This change just adds a link to the microsoft docs to help out the next guy.