docs(upgrade): mark upgrade apis as stable

modules/@angular/upgrade/src/upgrade_adapter.ts

@@ -95,7 +95,7 @@ var upgradeCount: number = 0;
  *
  * ```
  *
- * @experimental
+ * @stable
  */
 export class UpgradeAdapter {
   /* @internal */
@@ -566,7 +566,7 @@ function ng1ComponentDirective(info: ComponentInfo, idPrefix: string): Function
 /**
  * Use `UpgradeAdapterRef` to control a hybrid AngularJS v1 / Angular v2 application.
  *
- * @experimental
+ * @stable
  */
 export class UpgradeAdapterRef {
   /* @internal */

tools/public_api_guard/upgrade/index.d.ts

@@ -1,4 +1,4 @@
-/** @experimental */
+/** @stable */
 export declare class UpgradeAdapter {
     constructor(ng2AppModule: Type<any>);
     bootstrap(element: Element, modules?: any[], config?: angular.IAngularBootstrapConfig): UpgradeAdapterRef;
@@ -10,7 +10,7 @@ export declare class UpgradeAdapter {
     }): void;
 }
 
-/** @experimental */
+/** @stable */
 export declare class UpgradeAdapterRef {
     ng1Injector: angular.IInjectorService;
     ng1RootScope: angular.IRootScopeService;