message: docs: add documentation for app.deleteFormFields
diff: - [getFormFields](#getFormFields)
  - [addFormFields](#addFormFields)
  - [updateFormFields](#updateFormFields)
  +- [deleteFormFields](#deleteFormFields)
  ## Overview
  @@ -242,6 +243,28 @@ Updates the field settings of fields in a form of an App.
  - https://developer.kintone.io/hc/en-us/articles/115005507788
  +### deleteFormFields
  +
  +Deletes fields from a form of an App.
  +
  +#### Parameters
  +
  +| Name | Type | Required | Description |
  +| -------- | :--------------: | :------: | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
  +| app | Number or String | Yes | The App ID. |
  +| fields | Array\<String\> | Yes | The list of field codes of the fields to delete.<br />Up to 100 field codes can be specified. |
  +| revision | Number or String | Yes | Specify the revision number of the settings that will be deployed.<br />The request will fail if the revision number is not the latest revision.<br />The revision will not be checked if this parameter is ignored, or -1 is specified. |
  +
  +#### Returns
  +
  +| Name | Type | Description |
  +| -------- | :----: | ---------------------------------------- |
  +| revision | String | The revision number of the App settings. |
  +
  +#### Reference
  +
  +- https://developer.kintone.io/hc/en-us/articles/115005341187
  +
  ### getAppCustomize
  Gets the JavaScript and CSS Customization settings of an App.
  