doc improvements based on Jesse's comments

core/src/main/java/hudson/model/Queue.java

@@ -741,6 +741,8 @@ public class Queue extends ResourceController implements Saveable {
 
     /**
      * Returns the snapshot of all {@link LeftItem}s.
+     *
+     * @since 1.519
      */
     public Collection<LeftItem> getLeftItems() {
         return Collections.unmodifiableCollection(leftItems.asMap().values());
@@ -1444,6 +1446,7 @@ public class Queue extends ResourceController implements Saveable {
          *
          * @return
          *      URL that ends with '/'.
+         * @since 1.519
          */
         @Exported
         public String getUrl() {
@@ -1741,6 +1744,8 @@ public class Queue extends ResourceController implements Saveable {
     /**
      * {@link Item} in the {@link Queue#leftItems} stage. These are items that had left the queue
      * by either began executing or by getting cancelled.
+     *
+     * @since 1.519
      */
     public final static class LeftItem extends Item {
         public final WorkUnitContext outcome;

core/src/main/java/hudson/model/queue/WorkUnit.java

@@ -27,6 +27,8 @@ import hudson.model.Executor;
 import hudson.model.Queue;
 import hudson.model.Queue.Executable;
 import hudson.model.Queue.Task;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 import org.kohsuke.stapler.export.ExportedBean;
 
 /**
@@ -76,6 +78,10 @@ public final class WorkUnit {
         return executable;
     }
 
+    /**
+     * This method is only meant to be called internally by {@link Executor}.
+     */
+    @Restricted(NoExternalUse.class)
     public void setExecutable(Executable executable) {
         this.executable = executable;
     }