Improve b:tree documentation

[TheCoder4eu]

Improve docs with the processing of check and uncheck the items (not automatic)
implement the methods (processValueChecked and processValueUnchecked) from TreeNodeEventListener interface .
use these methods like below:

    public void processValueChecked(TreeNodeCheckedEvent arg0) {
        Node node = (DefaultNodeImpl) arg0.getNode();
        node.setChecked(true);

        TreeModelUtils.updateNodeById(getCategoriasModelTree(), node.getNodeId(), node);
    }

See #456 .

[asterd]

Done!