chore(drilldown): add documentation for `getParentChain`

lib/features/drilldown/DrilldownBreadcrumbs.js

@@ -22,17 +22,17 @@ export default function DrilldownBreadcrumbs(eventBus, elementRegistry, overlays
   container.appendChild(breadcrumbs);
 
   function updateBreadcrumbs(currentRoot) {
-    var parents = getParentChain(currentRoot);
+    var boParents = getBoParentChain(currentRoot);
 
-    var path = parents.map(function(parent) {
+    var path = boParents.map(function(parent) {
       var title = escapeHTML(parent.name || parent.id);
       var link = domify('<li><span class="bjs-crumb"><a title="' + title + '">' + title + '</a></span></li>');
 
       var parentPlane = canvas.findRoot(planeId(parent)) || canvas.findRoot(parent.id);
 
+      // when the root is a collaboration, the process does not have a corresponding
+      // element in the elementRegisty. Instead, we search for the corresponding participant
       if (!parentPlane && is(parent, 'bpmn:Process')) {
-
-        // root is a collaboration
         var participant = elementRegistry.find(function(element) {
           var bo = getBusinessObject(element);
           return bo && bo.processRef && bo.processRef === parent;
@@ -69,7 +69,16 @@ DrilldownBreadcrumbs.$inject = [ 'eventBus', 'elementRegistry', 'overlays', 'can
 
 
 // helpers
-function getParentChain(child) {
+
+/**
+ * Returns the parents for the element using the business object chain,
+ * starting with the root element.
+ *
+ * @param {djs.model.Shape} child
+ *
+ * @returns {Array<djs.model.Shape>} parents
+ */
+function getBoParentChain(child) {
   var bo = getBusinessObject(child);
 
   var parents = [];