Update hostdb api doc

@ChrisSchinnerl This PR updates the HostDB API documentation. This includes the change of LastHistoricUpdate to lower case but does not include the IPNets and LastIPNetChange fields as they are not live yet.

This PR is still WIP as I am struggling with a good description for the historicdowntime, historicuptime and lasthistoricupdate fields. Any suggestions what to put there?