Update documentation

docs/configuration-v3_4_experimental.md

@@ -156,7 +156,8 @@ The Ignition configuration is a JSON document conforming to the following specif
     * **_dropins_** (list of objects): the list of drop-ins for the unit. Every drop-in must have a unique `name`.
       * **name** (string): the name of the drop-in. This must be suffixed with ".conf".
       * **_contents_** (string): the contents of the drop-in.
-    * **_scope_** (string): Allow to define "user" or "system" (default) level systemd units.
+    * **_scope_** (string): Allow to define "user", "global" or "system" (default) level systemd units. cf : [systemd Documentation](https://www.freedesktop.org/software/systemd/man/systemctl.html#enable%20UNIT%E2%80%A6)
+    * **_users_**  (list of strings): The list of users concerned by the unit if scope is set to "user".
 * **_passwd_** (object): describes the desired additions to the passwd database.
   * **_users_** (list of objects): the list of accounts that shall exist. All users must have a unique `name`.
     * **name** (string): the username for the account.