Doc: Add note for subnet creation
Doc: Add note for subnet creation

Add note that subnet creation on a network that already has bound ports
not always results in automatic update of the port, and link to the
related Neutron documentation.

Change-Id: I1898d2ca3692ca05f51caed16b2a4814bc5556e3
Related-Bug: #1719806

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib