[Samba] Update Wiki for backup script

[James]

Hello,

     I notice the script expects to have $PATH  include 
'/usr/local/samba/bin'. Script will fail if it can't find tdbbackup. It 
was obvious to me but others may stumble. Maybe someone could add under 
the 'About the samba_backup script' section.

"/This requires that your whole samba installation is on one place (like 
/usr/local/samba/). This script assumes you have included 
'usr/local/samba/bin' as part of your path variable for root./"

The script will fail with mention of a status code. However looking 
through the script, I see no mention of what these codes mean. Thoughts? 
Thanks.

https://wiki.samba.org/index.php/Backup_and_restore_an_Samba_AD_DC


-- 
-James