backups: document structure of backupConfig

I keep forgetting this, let's write this up once and for all

src/mounts.js

@@ -63,6 +63,7 @@ function validateMountOptions(type, options) {
     }
 }
 
+// managed providers are those for which we setup systemd mount file
 function isManagedProvider(provider) {
     return provider === 'sshfs' || provider === 'cifs' || provider === 'nfs' || provider === 'ext4' || provider === 'xfs' || provider === 'disk';
 }