Document topBar options for Android's showAsAction (#5199)

Closes  #4001

lib/src/interfaces/Options.ts

    * Set testID for reference in E2E tests
    */
   testID?: string;
+  /**
+   * (Android only) Set showAsAction value
+   * @see {@link https://developer.android.com/guide/topics/resources/menu-resource|Android developer guide: Menu resource} 
+   */
+  showAsAction?: 'ifRoom' | 'withText' | 'always' | 'never';
 }
 
 export interface OptionsTopBar {