Document in the Kea ARM that Kea and database timezones must match

There was a user issue described in the #1968 which caused the Kea servers to refuse renewing leases when Kea server used a different timezone setting than the database timezone setting. I skimmed through our documentation and found no indication that they must match. I propose that we better document in the Kea ARM that the timezone settings between Kea and database must not differ. I also propose that we add some examples how it can be verified for different databases. This is purely a documentation change.