Doc: Refresh NDP proxy doc
Doc: Refresh NDP proxy doc

Actualize NDP proxy command for merged OSC, and rewrite some sentences.

Change-Id: I92ea4b63984fee1197dcda19d6ecc5608c0f650f

openstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron