docs(core): add API documentation to standalone-related classes (#45687)

This commit adds some internal documentation.

PR Close #45687

packages/core/src/render3/features/standalone_feature.ts

@@ -13,7 +13,11 @@ import {OnDestroy} from '../../interface/lifecycle_hooks';
 import {ComponentDef} from '../interfaces/definition';
 import {createEnvironmentInjector} from '../ng_module_ref';
 
-// TODO(pk): document
+/**
+ * A service used by the framework to create instances of standalone injectors. Those injectors are
+ * created on demand in case of dynamic component instantiation and contain ambient providers
+ * collected from the imports graph rooted at a given standalone component.
+ */
 class StandaloneService implements OnDestroy {
   cachedInjectors = new Map<ComponentDef<unknown>, EnvironmentInjector|null>();
 
@@ -60,7 +64,12 @@ class StandaloneService implements OnDestroy {
 }
 
 /**
- * TODO(pk): documentation
+ * A feature that acts as a setup code for the {@see StandaloneService}.
+ *
+ * The most important responsaibility of this feature is to expose the "getStandaloneInjector"
+ * function (an entry points to a standalone injector creation) on a component definition object. We
+ * go through the features infrastructure to make sure that the standalone injector creation logic
+ * is tree-shakable and not included in applications that don't use standalone components.
  *
  * @codeGenApi
  */

packages/core/src/render3/interfaces/definition.ts

@@ -322,6 +322,9 @@ export interface ComponentDef<T> extends DirectiveDef<T> {
    */
   tView: TView|null;
 
+  /**
+   * A function added by the {@see ɵɵStandaloneFeature} and used by the framework to create standalone injectors.
+   */
   getStandaloneInjector: ((parentInjector: EnvironmentInjector) => EnvironmentInjector | null)|null;
 
   /**