ug: offline-update: Add return code for downgrade attempt

Signed-off-by: Andre Detsch <andre.detsch@foundries.io>

source/user-guide/offline-update/offline-update.rst

@@ -161,6 +161,8 @@ The ``install`` command sets the following exit codes:
     - Reboot is required to complete installation. After reboot ``aklite-offline run`` must be invoked.
 - *101*: Installation succeeded.
     - Restart of dockerd service is required to complete installation, e.g. ``systemctl restart docker``. After the restart ``aklite-offline run`` must be invoked.
+- *102*: Downgrade attempt.
+    - The Target to be installed has a version lower than the one that is running. The ``--force`` option can be used to force its installation.
 
 The ``run`` command sets the following exit codes: