docs: add clarifications to move servers
docs: add clarifications to move servers

This adds in some titles, to replace the bold text that simulated
heading. It also changes the text into use cases rather than actions.

It includes a some additions around:
* resource optimization use cases for live-migration
* data loss during evacuate
* downtime during resize
* clearer description of why a user would call shelve

This does not complete this doc, but should help improve the readability.

blueprint complete-todo-in-api-concept-doc

Change-Id: Ib2cf479c14a1c2f3b119ba7f549a00c9bd541cf9

apporc/nova,Juniper/nova,BeyondTheClouds/nova,gooddata/openstack-nova,phenoxim/nova,vmturbo/nova,hanlind/nova,zhimin711/nova,mahak/nova,jianghuaw/nova,CEG-FYP-OpenStack/scheduler,cernops/nova,jianghuaw/nova,klmitch/nova,rajalokan/nova,bigswitch/nova,NeCTAR-RC/nova,NeCTAR-RC/nova,jianghuaw/nova,vmturbo/nova,hanlind/nova,klmitch/nova,mahak/nova,vmturbo/nova,sebrandon1/nova,cernops/nova,BeyondTheClouds/nova,mikalstill/nova,rajalokan/nova,alaski/nova,vmturbo/nova,phenoxim/nova,mahak/nova,sebrandon1/nova,cyx1231st/nova,hanlind/nova,mikalstill/nova,Juniper/nova,openstack/nova,dims/nova,openstack/nova,cyx1231st/nova,gooddata/openstack-nova,bigswitch/nova,klmitch/nova,gooddata/openstack-nova,BeyondTheClouds/nova,sebrandon1/nova,gooddata/openstack-nova,devendermishrajio/nova,alaski/nova,rahulunair/nova,cloudbase/nova,mikalstill/nova,rahulunair/nova,rahulunair/nova,devendermishrajio/nova,klmitch/nova,dims/nova,apporc/nova,openstack/nova,cernops/nova,rajalokan/nova,jianghuaw/nova,Juniper/nova,zhimin711/nova,cloudbase/nova,cloudbase/nova,rajalokan/nova,CEG-FYP-OpenStack/scheduler,Juniper/nova