add docs of iml backup in help page

check_clientbackup

@@ -43,6 +43,10 @@ Licence: GNU GPL 3
 ______________________________________________________________________
 
 Show status of IML backup.
+See https://os-docs.iml.unibe.ch/iml-backup/
+
+It returns with state UNKNOWN if no backup has been performed yet.
+It switches to ERROR an error occured during backup process.
 
 SYNTAX:
 $_self [-d DIRECTORY]