docs(plugins): add import statements for usage clarity

src/plugins/splashscreen.ts

@@ -5,6 +5,10 @@ import {Plugin, Cordova} from './plugin'
  * @description This plugin displays and hides a splash screen during application launch. The methods below allows showing and hiding the splashscreen after the app has loaded.
  * @usage
  * ```ts
+ * import {Splashscreen} from 'ionic-native';
+ *
+ *
+ *
  * Splashscreen.show();
  *
  * Splashscreen.hide();