[arch-guide] change to high-availability rst file in the arch guide draft
[arch-guide] change to high-availability rst file in the arch guide draft

made some small changes to the rst file
corrected and changed guarantees to guaranteed
added if/when instead of just when because it may not happen
changed neighbour to neighbor - consistency with all docs

Change-Id: Iffa75d88f5c7cfc23aac5c88809943420131c37e

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals