[Common UI] Document TreeNodeController

Add in-line documentation to TreeNodeController, and update
glossary with some clarifying definition.

Additionally, change name from tree-item to tree-node for
consistency.

Part of ongoing transition of commonUI bundles, WTD-574.

platform/commonUI/general/res/templates/tree.html

@@ -1,6 +1,6 @@
 <ul class="tree">
     <li ng-repeat="child in composition">
-        <mct-representation key="'tree-item'" mct-object="child" parameters="parameters">
+        <mct-representation key="'tree-node'" mct-object="child" parameters="parameters">
         </mct-representation>
     </li>
 </ul>