Improve documentation on how to use database storage for forensic logging hook

Based on a customer support ticket (#15209) our documentation on configuring database storage for the forensic logging hook (sections 15.6.4 and 15.6.5) could be improved by providing configuration examples in that section.

Wlodek was kind enough to provide a sample config from a system test but Wlodek doesn't scale for every Kea server operator. :-)

(until the documentation gets improved, here's the configuration snippet in case anyone comes looking through our issue tracker trying to find this information..)

"hooks-libraries":[
   {    
       "library":"libdhcp_legal_log.so",
       "parameters":
       {    
           "name":"kea",
           "password":"kea",
           "type":"mysql",
           "user":"kea"
       }    
            
   }    
]