docs(common): rewrite docs for NgForOf#ngForTrackBy (#42329)

Clarify the prupose of the tracking function and document how to create a good one.

Fixes #40461

PR Close #42329

packages/common/src/directives/ng_for_of.ts

@@ -141,21 +141,22 @@ export class NgForOf<T, U extends NgIterable<T> = NgIterable<T>> implements DoCh
     this._ngForOfDirty = true;
   }
   /**
-   * A function that defines how to track changes for items in the iterable.
+   * Specifies a custom `TrackByFunction` to compute the identity of items in an iterable.
    *
-   * When items are added, moved, or removed in the iterable,
-   * the directive must re-render the appropriate DOM nodes.
-   * To minimize churn in the DOM, only nodes that have changed
-   * are re-rendered.
+   * If a custom `TrackByFunction` is not provided, `NgForOf` will use the item's [object
+   * identity](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is)
+   * as the key.
    *
-   * By default, the change detector assumes that
-   * the object instance identifies the node in the iterable.
-   * When this function is supplied, the directive uses
-   * the result of calling this function to identify the item node,
-   * rather than the identity of the object itself.
+   * `NgForOf` uses the computed key to associate items in an iterable with DOM elements
+   * it produces for these items.
    *
-   * The function receives two inputs,
-   * the iteration index and the associated node data.
+   * A custom `TrackByFunction` is useful to provide good user experience in cases when items in an
+   * iterable rendered using `NgForOf` have a natural identifier (for example, custom ID or a
+   * primary key), and this iterable could be updated with new object instances that still
+   * represent the same underlying entity (for example, when data is re-fetched from the server,
+   * and the iterable is recreated and re-rendered, but most of the data is still the same).
+   *
+   * @see `TrackByFunction`
    */
   @Input()
   set ngForTrackBy(fn: TrackByFunction<T>) {

packages/core/src/change_detection/differs/iterable_differs.ts

@@ -113,14 +113,52 @@ export interface IterableChangeRecord<V> {
 }
 
 /**
- * An optional function passed into the `NgForOf` directive that defines how to track
- * changes for items in an iterable.
- * The function takes the iteration index and item ID.
- * When supplied, Angular tracks changes by the return value of the function.
+ * A function optionally passed into the `NgForOf` directive to customize how `NgForOf` uniquely
+ * identifies items in an iterable.
  *
+ * `NgForOf` needs to uniquely identify items in the iterable to correctly perform DOM updates
+ * when items in the iterable are reordered, new items are added, or existing items are removed.
+ *
+ *
+ * In all of these scenarios it is usually desirable to only update the DOM elements associated
+ * with the items affected by the change. This behavior is important to:
+ *
+ * - preserve any DOM-specific UI state (like cursor position, focus, text selection) when the
+ *   iterable is modified
+ * - enable animation of item addition, removal, and iterable reordering
+ *
+ * A common use for custom `trackBy` functions is when the model that `NgForOf` iterates over
+ * contains a property with a unique identifier. For example, given a model:
+ *
+ * ```ts
+ * class User {
+ *   id: number;
+ *   name: string;
+ *   ...
+ * }
+ * ```
+ * a custom `trackBy` function could look like the following:
+ * ```ts
+ * function userTrackBy(index, user) {
+ *   return user.id;
+ * }
+ * ```
+ *
+ * A custom `trackBy` function must have several properties:
+ *
+ * - be [idempotent](https://en.wikipedia.org/wiki/Idempotence) (be without side effects, and always
+ * return the same value for a given input)
+ * - return unique value for all unique inputs
+ * - be fast
+ *
+ * @see [`NgForOf#ngForTrackBy`](api/common/NgForOf#ngForTrackBy)
  * @publicApi
  */
 export interface TrackByFunction<T> {
+  /**
+   * @param index The index of the item within the iterable.
+   * @param item The item in the iterable.
+   */
   (index: number, item: T): any;
 }