docs(plugins): add import statements for usage clarity

src/plugins/batterystatus.ts

@@ -8,6 +8,10 @@ import {Observable} from "rxjs/Observable";
  *
  * @usage
  * ```js
+ * import {BatteryStatus} from 'ionic-native';
+ *
+ *
+ *
  * // watch change in battery status
  * let subscription = BatteryStatus.onChange().subscribe(
  *  status => {