Improve AMT driver doc
Improve AMT driver doc

It is recommended that users should deploy AMT nodes with local boot.

Co-Authored-By: Chris Krelle <nobodycam@gmail.com>

Close-Bug: #1436824
Change-Id: I536c587008bb68950ec7acfcf16c8ae28f9560bc

openstack/ironic,SauloAislan/ironic,hpproliant/ironic,pshchelo/ironic,naterh/ironic,NaohiroTamura/ironic,pshchelo/ironic,naototty/vagrant-lxc-ironic,redhat-openstack/ironic,ionutbalutoiu/ironic,naototty/vagrant-lxc-ironic,supermari0/ironic,devananda/ironic,ionutbalutoiu/ironic,Tan0/ironic,bacaldwell/ironic,NaohiroTamura/ironic,dims/ironic,dims/ironic,rdo-management/ironic,bacaldwell/ironic,openstack/ironic,SauloAislan/ironic