We have a page at https://www.zabbix.com/documentation/2.4/manual/appendix/items/vm.memory.size_params that lists available parameters for vm.memory.size key.
However, some descriptions are very platform-specific. For instance, the following is correct on BSD systems only, even though "available" is supported on every platform:
used - active + wired memory
available - inactive + cached + free memory
This page should probably be improved so that it describes the platforms on which a particular parameter is supported and it should describe the meaning of a particular parameter on a particular platform.