Clarify Sync Gateway Resync requirements

The linked page's final paragraph attempts to explain the state that a Sync Gateway environment should be in before a resync can be performed. These should be clarified as the wording is a little confusing. The following steps are:

1. Bring all nodes offline
2. Issue _resync on one
3. Wait until done
4. Bring back all nodes online

Main thing to bring to attention is that starting this action on more than one node would result in multiple resyncs running and the system behavior will be undefined. In a multi-node cluster, the resync must be only run on one node whilst all nodes are offline.