message: flesh out better <Details> docs
diff: # Details
  -```.jsx
  -<ExampleHeading>With static children</ExampleHeading>
  +The Details component is an HTML `<details>` element without native browser styling that optionally uses the [render props pattern](https://reactjs.org/docs/render-props.html) to pass its state to child components.
  +
  +You are responsible for rendering your own `<summary>`. To style your summary element like a [Button](./Button), you can use the `is` prop:
  +
  +```jsx
  +<Button is="summary">Summary text</Button>
  +```
  +## With static children
  +```.jsx
  <Details>
  - <summary className="btn">Click me</summary>
  + <Button is="summary">Click me</Button>
  <p>This should show and hide</p>
  </Details>
  +```
  +## With children as a function
  +The render function gets an object with two keys:
  -<ExampleHeading>With children as a function</ExampleHeading>
  +* `open` is a boolean reflecting the `<details>` element's `open` attribute, and can be used to conditionally show or hide content.
  +* `toggle` is a function that can be assigned to event handlers to trigger toggling of the `open` state.
  +If you use this form or the render prop (see below), **you must attach the `toggle` prop as an event listener**. If you don't the render function will not be called when the element is toggled by the native browser behavior.
  +
  +```.jsx
  <Details>
  {({open, toggle}) => (
  - <React.Fragment>
  - <summary className="btn" onClick={toggle}>
  + <>
  + <Button is="summary" onClick={toggle}>
  {open ? 'Hide' : 'Show'}
  - </summary>
  + </Button>
  <p>This should show and hide</p>
  - </React.Fragment>
  + </>
  )}
  </Details>
  +```
  -<ExampleHeading>With render prop</ExampleHeading>
  -<Details render={() => 'hi'} />
  +## With render prop
  +The Details component also accepts a `render` function prop.
  +
  +```.jsx
  +<Details render={({open, toggle}) => (
  + <>
  + <Button is="summary" onClick={toggle}>Open? {String(open)}</Button>
  + <p>This is the content.</p>
  + </>
  +)} />
  ```
  ## System props
  