Update docs/configuring-playbook-backup-borg.md: add the common section "Troubleshooting"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-backup-borg.md

@@ -145,3 +145,7 @@ Sometimes it can be helpful to run the backup as you'd like, avoiding to wait un
 If you want to run it immediately, log in to the server with SSH and run `systemctl start matrix-backup-borg`.
 
 This will not return until the backup is done, so it can possibly take a long time. Consider using [tmux](https://en.wikipedia.org/wiki/Tmux) if your SSH connection is unstable.
+
+## Troubleshooting
+
+As with all other services, you can find the logs in [systemd-journald](https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html) by logging in to the server with SSH and running `journalctl -fu backup-borg`.