docs: add note that when ngOnInit not used (angular#34619)

In some examples ngOnInit was missing added note to tell when it is missing.

Fixes angular#34562

PR Close angular#34619

aio/content/start/index.md

@@ -311,6 +311,13 @@ To make the "Notify Me" button work, you need to configure two things:
 
 1. In the component class, define a property named `notify` with an `@Output()` decorator and an instance of `EventEmitter()`. This allows the product alert component to emit an event when the value of the notify property changes.
 
+<div class="alert is-helpful">
+
+  When the Angular CLI generates a new component, it includes an empty constructor, the `OnInit` interface, and the `ngOnInit()` method.
+  Since the following example isn't using them, they are omitted here for brevity.
+
+</div>
+
     <code-example path="getting-started/src/app/product-alerts/product-alerts.component.ts" header="src/app/product-alerts/product-alerts.component.ts" region="input-output"></code-example>
 
 1. In the product alert template, `product-alerts.component.html`, update the "Notify Me" button with an event binding to call the `notify.emit()` method.