Document DefaultQueueOp

Change-Id: Ia4cbd2e90a66ea9277a78c833a4a3da655e5eb8e
diff --git a/java/com/google/gerrit/server/git/DefaultQueueOp.java b/java/com/google/gerrit/server/git/DefaultQueueOp.java
index b30acfa..9fb1a9b 100644
--- a/java/com/google/gerrit/server/git/DefaultQueueOp.java
+++ b/java/com/google/gerrit/server/git/DefaultQueueOp.java
@@ -17,6 +17,10 @@
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
+/**
+ * Wrapper class so a Runnable can schedule itself onto the Gerrit Workqueue. Subclasses must
+ * implement the {@code run} method.
+ */
 public abstract class DefaultQueueOp implements Runnable {
   private final WorkQueue workQueue;