message: docs: Include section on textareas in `Form` docs
  Closes
diff: @@ -66,6 +66,54 @@ function Example() {
  }
  ```
  +### Textareas
  +
  +If your form requires a `textarea` instead of an `input` field, you can use the `as` prop on the `FormInput` component.
  +
  +<!-- eslint-disable no-alert -->
  +```jsx
  +import {
  + unstable_useFormState as useFormState,
  + unstable_Form as Form,
  + unstable_FormLabel as FormLabel,
  + unstable_FormInput as FormInput,
  + unstable_FormMessage as FormMessage,
  + unstable_FormSubmitButton as FormSubmitButton
  +} from "reakit/Form";
  +
  +function Example() {
  + const form = useFormState({
  + values: { message: "" },
  + onValidate: values => {
  + if (!values.message) {
  + const errors = {
  + message: "Please enter a message."
  + };
  + throw errors;
  + }
  + },
  + onSubmit: values => {
  + alert(JSON.stringify(values, null, 2));
  + }
  + });
  + return (
  + <Form {...form}>
  + <FormLabel {...form} name="message">
  + Message
  + </FormLabel>
  + <FormInput
  + {...form}
  + name="message"
  + placeholder="What's on your mind?"
  + as="textarea"
  + />
  + <FormMessage {...form} name="message" />
  + <FormSubmitButton {...form}>Submit</FormSubmitButton>
  + </Form>
  + );
  +}
  +```
  +
  ### Arrays
  `Form` supports array values seamlessly. For convenience, you can reliably use the array indexes as keys on the array fragments.
  