[networking] Describe existing security group behavior
[networking] Describe existing security group behavior

Providing a more complete description of the existing security group
behavior. This should help operators and user better understand what
traffic is permitted by security groups, and should help developers
implementing additional firewall drivers to achieve consistent behavior.

backport: mitaka
Co-Authored-By: Matt Kassawara <4c2e6149ef19dba98a7fe8873ce28b2098aceda7@gmail.com>
Change-Id: I8cc078161dfbf649f466233230eaf6e4ece0429d

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals