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';
 }

src/routes/backups.js

@@ -119,6 +119,16 @@ async function setLimits(req, res, next) {
     next(new HttpSuccess(200, {}));
 }
 
+// storage has three parts. these fields are merged into one top level object
+// 1. format. rsync or tgz
+// 2. config. the 'provider' (see api() function in src/storage.js) differentiates further options
+//      s3 providers - accessKeyId, secretAccessKey, bucket, prefix etc . see s3.js
+//      gcs - bucket, prefix, projectId, credentials . see gcs.js
+//      ext4/xfs/disk (managed providers) - mountOptions (diskPath), prefix, noHardlinks. disk is legacy.
+//      nfs/cifs/sshfs (managed providers) - mountOptions (host/username/password/seal/privateKey etc), prefix, noHardlinks
+//      filesystem - backupFolder, noHardlinks
+//      mountpoint - mountPoint, prefix, noHardlinks
+// 3. encryption. password and encryptedFilenames
 async function setStorage(req, res, next) {
     assert.strictEqual(typeof req.body, 'object');