fix comment in DCReference

Couple of issues:
1. The following comment is incorrect in DCTree.java
   public static class DCReference extends DCEndPosTree<DCReference> implements ReferenceTree {
        public final String signature;

        // The following are not directly exposed through ReferenceTree
        // use DocTrees.getElement(TreePath,ReferenceTree)
        public final JCTree qualifierExpression;

 DocTrees.getElement(TreePath,ReferenceTree) does not exist.

DocTrees.getElement(TreePath) exists albeit it returns the wrong element ie. the holder or owner
of the DocTree.

The following are the steps to extract the TypeElement, of a ThrowsTree's thrown TypeElement
such as java.io.IOException
    ReferenceTree exceptionName = ((ThrowsTree) dtree).getExceptionName();
    DocTreePath path = DocTreePath.getPath(path, docCommentTree, exceptionName);
    System.out.println((TypeElement) instance.getElement(path));