Documentation about chains' priorities, lists of elements etc.

Documentation about chains' priorities, lists of elements etc.

This patch adds several aspects of documentation about the network filtering
system:

- chains, chains' priorities and chains' default priorities
- talks about lists of elements, i.e., a variable assigned multiple values
  (part of already ACK-ed series)
- already mentions the vlan, stp and mac chains added later on
  (https://www.redhat.com/archives/libvir-list/2011-October/msg01238.html)
- mentions limitations of vlan filtering (when sent by VM) on Linux systems