vm.memory.size[available] described in "Upgraede note for 3.0.0" below.

vm.memory.size[available] item on Linux now reads MemAvailable (the system's native estimate) from /proc/meminfo on Linux kernels 3.14 and above.
Depending on the system state (like the amount of reclaimable slab), the new value may differ significantly from the old version and may thus cause triggers to change their value.
Before Zabbix 3.0 it was always the sum of free, buffers, and cached memories.

And the following on this, It says "free + buffers" on available with Linux<3.14

• 3.0 : vm.memory.size parameters
• 3.2 : vm.memory.size parameters
• 3.4 : vm.memory.size parameters
• 4.0 : vm.memory.size parameters

Just wondering, in documentation for 2.4 few lines are added:

• on Linux available is "free + buffers + cached"

a minor edit and opinion,
This explain better to change "free + buffers" => "free + buffers + cached"