Add note about default behavior of server backup

Both mysqldump and mysqlbackup providers by default back up all databases in the database (i.e. when an empty array is specfied).
This patch documents that behavior.
See templates/xtrabackup.sh.erb and templates/mysqlbackup.sh.erb

Author: jacksgt

REFERENCE.md

@@ -740,9 +740,9 @@ Default value: `false`
 
 Data type: `Any`
 
-Databases to backup (if using xtrabackup provider).
+Databases to backup (required if using xtrabackup provider).
 
-Default value: []
+Default value: `[]` (backs up all databases)
 
 ##### `file_per_database`