Add a comment explaining why we need to walk up the tree

Author: mleibman

src/cdk/a11y/focus-monitor/focus-monitor.ts

@@ -139,6 +139,7 @@ export class FocusMonitor implements OnDestroy {
    */
   private _documentFocusAndBlurListener = (event: FocusEvent) => {
     const target = event.target as HTMLElement|null;
+    // We need to walk up the ancestor chain in order to support `checkChildren`.
     for (let el = target; el; el = el.parentElement) {
       if (event.type === 'focus') {
         this._onFocus(event as FocusEvent, el);