Describe a little better what getRuntimeHint does

Change-Id: Iea0a00b6809488378038177a0213fe02b6063f10
diff --git a/src/com/android/tradefed/testtype/IRuntimeHintProvider.java b/src/com/android/tradefed/testtype/IRuntimeHintProvider.java
index f60dadd..f79db79 100644
--- a/src/com/android/tradefed/testtype/IRuntimeHintProvider.java
+++ b/src/com/android/tradefed/testtype/IRuntimeHintProvider.java
@@ -19,7 +19,10 @@
 public interface IRuntimeHintProvider {
 
     /**
-     * Give a hint about the test's runtime. Default is 1 minute.
+     * Return the expected runtime of the test in
+     * milliseconds.
+     *
+     * The time is used to load balance sharded execution
      */
     long getRuntimeHint();
 }