message: docs: add note on addApp document
diff: @@ -567,6 +567,9 @@ Creates a preview App.
  | name | String | Yes | The App name.<br />The maximum length is 64 characters. |
  | space | Number or String | | The Space ID of where the App will be created. |
  +Although `thread` parameter is required in [Add Preview App API](https://developer.kintone.io/hc/en-us/articles/115004712547), there are no visual representations of apps being related to threads.
  +Moreover, there is currently no helpful reason to create apps in threads other than the default thread, so the API Client sets the default thread id here.
  +
  #### Returns
  | Name | Type | Description |
  