Add error key to form description.mdx (#1063)

Author: maurer2

src/content/docs/useform/form.mdx

@@ -40,7 +40,7 @@ All props are optional
 | `render`         | <TypeText>`Function`</TypeText>                    | Render prop function suitable for headless component.                                                                   | <CodeArea withOutCopy rawData="<Form render={({ submit }) => <View/>} />" />                                       |
 | `onSubmit`       | <TypeText>`Function`</TypeText>                    | Function invoked after successful validation.                                                                           | <CodeArea withOutCopy rawData="<Form onSubmit={({ data }) => mutation(data)} />"/>                                 |
 | `onSuccess`      | <TypeText>`Function`</TypeText>                    | Function called after successful request to the server.                                                                 | <CodeArea withOutCopy rawData="<Form onSuccess={({ response }) => {}} />" />                                       |
-| `onError`        | <TypeText>`Function`</TypeText>                    | Function called after failed request to the server.<br/><br/>`setError` function will be called to update errors state. | <CodeArea withOutCopy rawData="<Form onError={({ response }) => {}} />" />                                         |
+| `onError`        | <TypeText>`Function`</TypeText>                    | Function called after failed request to the server.<br/><br/>`setError` function will be called to update errors state. `root.server` will be used as error key. | <CodeArea withOutCopy rawData="<Form onError={({ response }) => {}} />" />                                         |
 | `headers`        | <TypeText>`Record<string, string>`</TypeText>      | Request headers object.                                                                                                 | <CodeArea withOutCopy rawData="<Form headers={{ accessToken:  'xxx', 'Content-Type':  'application/json'  }} />"/> |
 | `validateStatus` | <TypeText>`(status: number) => boolean`</TypeText> | Function to validate status code.                                                                                       | <CodeArea withOutCopy rawData="<Form validateStatus={status => status === 200} />" />                              |