document that none of the "nova-manage network modify" commands should be used w/Quantum

[dan wendlandt]

danwent@ubuntu:~/nova$ ./bin/nova-manage network modify --help
Usage: nova-manage network modify <args> [options]

Options:
  -h, --help show this help message and exit
  --fixed_range=<x.x.x.x/yy>
                        Network to modify
  --project=<project name>
                        Project name to associate
  --host=<host> Host to associate
  --disassociate-project
                        Disassociate Network from Project
  --disassociate-host Disassociate Host from Project