doc improvement.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@5320 71c3de6d-444a-0410-be80-ed276b4c234a

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

@@ -40,6 +40,10 @@ public class ResourceController {
         }
     };
 
+    /**
+     * Union of all {@link Resource}s that are currently in use.
+     * Updated as a task starts/completes executing. 
+     */
     private ResourceList inUse = ResourceList.EMPTY;
 
     /**