docs(core): update stability markers for core apis

modules/@angular/core/src/linker/ng_module_factory.ts

@@ -21,7 +21,7 @@ import {CodegenComponentFactoryResolver, ComponentFactoryResolver} from './compo
  * `NgModuleRef` provides access to the NgModule Instance as well other objects related to this
  * NgModule Instance.
  *
- * @experimental
+ * @stable
  */
 export abstract class NgModuleRef<T> {
   /**

modules/@angular/core/src/linker/ng_module_factory_loader.ts

@@ -10,7 +10,7 @@ import {NgModuleFactory} from './ng_module_factory';
 
 /**
  * Used to load ng module factories.
- * @experimental
+ * @stable
  */
 export abstract class NgModuleFactoryLoader {
   abstract load(path: string): Promise<NgModuleFactory<any>>;