Update nodes-shutdown.asciidoc

This is submitted per user feedback from the field.  Some admins have trouble locating documentation on the proper way to restart a cluster because such information is currently embedded within the steps in upgrade guide (http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/setup-upgrade.html#rolling-upgrades).  It will be helpful to have a separate reference section on how to properly restart a cluster (not in the context of an upgrade).  This is an attempt to create such a section - not sure if this is the right location for it, so feel free to move it to a different location where appropriate.