Add time unit to Limit#onSample (#228)

Author: Meijuh

concurrency-limits-core/src/main/java/com/netflix/concurrency/limits/Limit.java

@@ -35,8 +35,8 @@ public interface Limit {
 
     /**
      * Update the limiter with a sample
-     * @param startTime
-     * @param rtt
+     * @param startTime start time in nanoseconds
+     * @param rtt round trip time in nanoseconds
      * @param inflight
      * @param didDrop
      */