Document lack of expiration propagation

Ref nextcloud/server#25320 for Nextcloud 22

Author: LukasReschke

user_manual/files/sharing.rst

@@ -40,6 +40,7 @@ For public *file* shares, you may allow editing the file with one of Nextcloud's
 
 .. figure:: ../images/sharing_public_file.png
 
+.. note:: Password protection as well as file expiration are not propagated using Federated File Sharing in current Nextcloud releases. This will be adjusted in Nextcloud 22.
 
 Internal shares with users and groups
 -------------------------------------