Specify difference between NSM and NSM dummy in docmentation

Specify difference between NSM and NSM dummy in docmentation

Explicitly states the difference between the Node State Manager Dummy
and the real Node State Manager, i.e.
* The NSM dummy toggles between successfully setting the node state
  and failing to set the node state.
* The NSM dummy toggles between the LUC being required, and the LUC
  not being required.
* The NSM dummy will shut down its fast shutdown clients, then its
  normal shutdown clients, on receiving the SIGHUP signal