We have UnavailableDelay, UnreachableDelay, UnreachablePeriod descriptions here:
http://www.zabbix.com/documentation/2.0/manual/appendix/config/zabbix_server

I recall it was not easy to me (in the past) to understand what is what and relations between them.
I'm getting now similar questions from users.

This means it's really not clear and we have to explain it better in doc, somehow

Some details available here:
http://www.zabbix.com/documentation/1.8/manual/about/what_s_new_1.8.9#zabbix_server_improvements
Take into account also:
https://support.zabbix.com/browse/ZBX-4284 - there are some technical details as well.

Suggested almost copy-paste from the what_s_new_1.8.9 page and maybe add some more explanation.
Also relations between UnreachableDelay and UnreachablePeriod - suggested to explain that logically UnreachableDelay should be several times bigger than UnreachablePeriod
and UnavailableDelay logically should be bigger than UnreachablePeriod

Suggested place is http://www.zabbix.com/documentation/2.0/manual/appendix/items, probably separate page will be required.
Then add internal link to this page from the "config/zabbix_server" page

It's not clear that such settings related only to agent, snmp, ipmi, jmx checks - should be described.
Ask devs before describe it.