Add description about latency annotation

Change-Id: I91d831ff2ce67a1cc8d762f7f59a7ee01a6f1718
diff --git a/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java b/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
index 8d2a73a..4101738 100644
--- a/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
+++ b/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
@@ -101,6 +101,7 @@
 
     /**
      * Annotation key for latency.
+     * The value of this key is expected to be latency in microsecond.
      */
     public static final String LATENCY = "latency";