Disambiguation of the difference between reachability and availability.

Change-Id: Ie4c7468453d64a48a0bf712737f927f7086a5bc5

@@ -50,6 +50,8 @@ public interface DeviceProvider extends Provider {
 
     /**
      * Checks the reachability (connectivity) of a device from this provider.
+     * Reachability, unlike availability, denotes whether THIS particular node
+     * can send messages and receive replies from the specified device.
      *
      * @param deviceId  device identifier
      * @return true if reachable, false otherwise

@@ -123,6 +123,10 @@ public interface DeviceService
 
     /**
      * Indicates whether or not the device is presently online and available.
+     * Availability, unlike reachability, denotes whether ANY node in the
+     * cluster can discover that this device is in an operational state,
+     * this does not necessarily mean that there exists a node that can
+     * control this device.
      *
      * @param deviceId device identifier
      * @return true if the device is available