Disambiguation of the difference between reachability and availability.

Change-Id: Ie4c7468453d64a48a0bf712737f927f7086a5bc5
diff --git a/core/api/src/main/java/org/onosproject/net/device/DeviceService.java b/core/api/src/main/java/org/onosproject/net/device/DeviceService.java
index 4e227ac..e6cd2d7 100644
--- a/core/api/src/main/java/org/onosproject/net/device/DeviceService.java
+++ b/core/api/src/main/java/org/onosproject/net/device/DeviceService.java
@@ -123,6 +123,10 @@
 
     /**
      * 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