Fix javadoc for QueueTaskDispatcher and NodeProperty

There is a BuildableItem interface in hudson.model, but the references
in these files to BuildableItem objects are refering to an inner class
of the Queue object, not to the interface defined in huson.model.  The
javadoc link improvement may help authors see that difference.
Merge pull request #878 from MarkEWaite/javadoc-fixes-e

Fix javadoc for QueueTaskDispatcher and NodeProperty