link page for unavailable hosts to internal item zabbix[host,<type>,available] description

For item zabbix[host,<type>,available] (on page
https://www.zabbix.com/documentation/2.2/manual/config/items/itemtypes/internal) create a link to https://www.zabbix.com/documentation/2.2/manual/appendix/items/unreachability#unavailable_host

It should help zabbix users to understand that increased UnreachablePeriod will lead to delayed actual values for the key.