WorkQueue: Add comment on additional constructor

Add a comment to clarify why the additional constructor is needed, to
avoid anyone attempting to remove it in future.

Change-Id: I724bb87409d39b30ac699299b7571b956cd0fddb

gerrit-server/src/main/java/com/google/gerrit/server/git/WorkQueue.java

@@ -92,6 +92,7 @@ public class WorkQueue {
     this(idGenerator, cfg.getInt("execution", "defaultThreadPoolSize", 1));
   }
 
+  /** Constructor to allow binding the WorkQueue more explicitly in a vhost setup. */
   public WorkQueue(IdGenerator idGenerator, int defaultThreadPoolSize) {
     this.idGenerator = idGenerator;
     this.queues = new CopyOnWriteArrayList<>();