docs: better backup docs

cli/app/backup.go

@@ -48,13 +48,27 @@ var appBackupCommand = cli.Command{
 This command runs an app backup.
 
 A backup command and pre/post hook commands are defined in the recipe
-configuration. Abra reads this config and run the comands in the context of the
-service. Pass <service> if you only want to back up a single service. All
-backups are placed in the ~/.abra/backups directory.
+configuration. Abra reads this configuration and run the comands in the context
+of the deployed services. Pass <service> if you only want to back up a single
+service. All backups are placed in the ~/.abra/backups directory.
 
-Example:
+A single backup file is produced for all backup paths specified for a service.
+If we have the following backup configuration:
 
-    abra app backup example.com db
+    - "backupbot.backup.path=/var/lib/foo,/var/lib/bar"
+
+And we run "abra app backup example.com app", Abra will produce a file that
+looks like:
+
+    ~/.abra/backups/example_com_app_609341138.tar.gz
+
+This file is a compressed archive which contains all backup paths. To see paths, run:
+
+    tar -tf ~/.abra/backups/example_com_app_609341138.tar.gz
+
+(Make sure to change the name of the backup file)
+
+This single file can be used to restore your app. See "abra app restore" for more.
 `,
 	Action: func(c *cli.Context) error {
 		app := internal.ValidateApp(c)