docs: add information on when not to use tree-shakable providers

aio/content/guide/dependency-injection.md

@@ -440,6 +440,12 @@ The service can be instantiated by configuring a factory function as shown below
 
 <code-example path="dependency-injection/src/app/tree-shaking/service.0.ts"  title="src/app/tree-shaking/service.0.ts" linenums="false"> </code-example>
 
+<div class="l-sub-section">
+
+Note that you cannot override tree-shakable providers registered in the `@injectable` decorator of the service. Use the `providers[]` array syntax to register providers when you intend to override providers in your app.
+
+</div>
+
 {@a injector-config} 
 {@a bootstrap}