[2.0, docs] DHCP start command documentation wrong for 2.0

Documentation bug.

The following commands are non-functional in maas 2.0. There is no node-groups. There is no node-group-interface.

$ uuid=$(maas <profile> node-groups list | grep uuid | cut -d\" -f4)

Once we have the UUID we can use this to update the node-group-interface for that nodegroup, and pass it the relevant interface details:

$ maas <profile> node-group-interface update $uuid eth0 \
        ip_range_high=192.168.123.200 \
        ip_range_low=192.168.123.100 \
        management=2 \
        broadcast_ip=192.168.123.255 \
        router_ip=192.168.123.1 \

dpkg -l:
ii maas 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all "Metal as a Service" is a physical cloud and IPAM
ii maas-cli 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS client and command-line interface
un maas-cluster-controller <none> <none> (no description available)
ii maas-common 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS server common files
ii maas-dhcp 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS DHCP server
ii maas-dns 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS DNS server
ii maas-proxy 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS Caching Proxy
ii maas-rack-controller 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all Rack Controller for MAAS
ii maas-region-api 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all Region controller API service for MAAS
ii maas-region-controller 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all Region Controller for MAAS
un maas-region-controller-min <none> <none> (no description available)
un python-django-maas <none> <none> (no description available)
un python-maas-client <none> <none> (no description available)
un python-maas-provisioningserver <none> <none> (no description available)
ii python3-django-maas 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS server Django web framework (Python 3)
ii python3-maas-client 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 2.0.0~beta7+bzr5112-0ubuntu1~xenial1 all MAAS server provisioning libraries (Python 3)

Expected result: MAAS Commands in documentation will match commands available.