[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-node.html

@@ -0,0 +1,27 @@
+<span ng-controller="ToggleController as toggle">
+    <span ng-controller="TreeNodeController as treeNode">
+        <span class="tree-item menus-to-left">
+            <span class='ui-symbol view-control'
+                  ng-click="toggle.toggle(); treeNode.trackExpansion()"
+                  ng-if="model.composition !== undefined">
+                {{toggle.isActive() ? "v" : ">"}}
+            </span>
+            <mct-representation key="'label'"
+                                mct-object="domainObject"
+                                parameters="parameters"
+                                ng-click="parameters.callback(domainObject)"
+                                ng-class="{selected: treeNode.isNavigated()}">
+            </mct-representation>
+        </span>
+        <span class="tree-item-subtree"
+              ng-show="toggle.isActive()"
+              ng-if="model.composition !== undefined">
+
+            <mct-representation key="'tree'"
+                                parameters="parameters"
+                                mct-object="treeNode.hasBeenExpanded() && domainObject">
+            </mct-representation>
+
+        </span>
+    </span>
+</span>