Mention limitation of borgmatic extract

[Felix Kleinekathöfer]

I ran into an issue while restoring to a backup using borgmatic as described in the docs (documented in a community post). Basically this issue occured because of a limitation of borg backup:

"borg extract only supports restoring into an empty destination. After that, the destination will exactly have the contents of the extracted archive. If you extract into a non-empty destination, borg will (for example) not remove files which are in the destination, but not in the archive." (source)

As of the mailcow docs restoring the maildir is done using

docker-compose exec borgmatic-mailcow borgmatic extract --path mnt/source --archive latest

. This makes it important to mention the limitation, otherwise duplicates or similar may occur.