Follow-up to rolling upgrade docs
Follow-up to rolling upgrade docs

This commit makes changes based upon review feedback
in If7f41ccbfd8d3128a142cd6e4f090c5c9825cf60 and discussion
in IRC around the time when that change landed, regarding
nodes being in stable states.

Change-Id: I5cb8a0419f63cb0ea0e19a4f1a1870fa70aae55c

pshchelo/ironic,openstack/ironic,openstack/ironic,pshchelo/ironic