Updating upgrade guide documentation
Updating upgrade guide documentation

This change updates the content and grammar of the upgrade guide
document.

Change-Id: I219457c16f935c7cd77b7e621b538be995700060

zhangjunli177/sahara,openstack/sahara,bigfootproject/sahara,zhujzhuo/Sahara,henaras/sahara,crobby/sahara,esikachev/sahara-backup,ekasitk/sahara,egafford/sahara,zhangjunli177/sahara,tellesnobrega/sahara,henaras/sahara,matips/iosr-2015,bigfootproject/sahara,tellesnobrega/sahara,zhujzhuo/Sahara,matips/iosr-2015,zhangjunli177/sahara,zhujzhuo/Sahara,redhat-openstack/sahara,henaras/sahara,bigfootproject/sahara,esikachev/sahara-backup,crobby/sahara,ekasitk/sahara,redhat-openstack/sahara,ekasitk/sahara,crobby/sahara,esikachev/sahara-backup,openstack/sahara,matips/iosr-2015,egafford/sahara,redhat-openstack/sahara