restore: document third-party repos (#531)

Restore: document third-party repos

Co-authored-by: Davide Principi <davide.principi@nethesis.it>
Co-authored-by: Filippo Carletti <filippo.carletti@gmail.com>

administrator-manual/en/disaster_recovery.rst

@@ -19,6 +19,14 @@ Other restored configurations:
    installation, i.e., avoid restoring a configuration backup from a 7.4.1708 installation on a new 7.6.1810 system,
    as it may lead to unexpected results.
 
+.. only:: nscom
+
+   .. note::
+
+       Third-party repositories are not restored by the disaster recovery procedure.
+       If the original machine has some third-party repositories enabled, remember to install them before
+       proceeding with the restore.
+
 New Server Manager
 ------------------