message: Explain `ref` needed for MenuButton `as` Component
  This just cost me half an hour!
  Could be explained more succinctly, and maybe in a better place in the document.
diff: @@ -85,6 +85,21 @@ To access the internal state of the `Menu`, use a function as `children`
  </Menu>
  ```
  +### Customising the button
  +
  +The default `MenuButton` can be styled using the usual styled-system props, but it
  +starts off plainly styled.
  +
  +Using the `as` prop of the `MenuButton`, you can render a custom component instead
  +of the default `MenuButton`. For instance, you can use Chakra's `Button` component,
  +or your own custom component.
  +
  +> Custom components must take a `ref` prop which is assigned to the React component
  +that triggers the menu opening. This is so that the `MenuList` popover can be
  +positioned correctly. Without this, the `MenuList` will render in an undefined
  +position.
  +
  +
  ### Letter Navigation
  When focus is on the `MenuButton` or within the `MenuList` and you type a letter
  