docs(material/tree): add a description of the when predicate (#21545)

- update TODO comments since issue was closed as a duplicate,
  point to the new issue

Author: Splaktar

src/material/tree/node.ts

@@ -82,6 +82,7 @@ export class MatTreeNode<T, K = T> extends _MatTreeNodeMixinBase<T, K>
 
 /**
  * Wrapper for the CdkTree node definition with Material design styles.
+ * Captures the node's template and a when predicate that describes when this node should be used.
  */
 @Directive({
   selector: '[matTreeNodeDef]',
@@ -139,9 +140,9 @@ export class MatNestedTreeNode<T, K = T> extends CdkNestedTreeNode<T, K>
     this._elementRef.nativeElement.classList.add('mat-nested-tree-node');
   }
 
-  // This is a workaround for https://github.com/angular/angular/issues/23091
+  // This is a workaround for https://github.com/angular/angular/issues/19145
   // In aot mode, the lifecycle hooks from parent class are not called.
-  // TODO(tinayuangao): Remove when the angular issue #23091 is fixed
+  // TODO(tinayuangao): Remove when the angular issue #19145 is fixed
   ngOnInit() {
     super.ngOnInit();
   }