message: add menu context docs
diff: @@ -40,6 +40,42 @@ SelectMenu components get `COMMON` system props. Read our [System Props](/system
  | :- | :- | :-: | :- |
  | initialTab | String | | If using the `SelectMenu.Tabs` component, you can use this prop to change the tab shown on open. By default, the first tab will be used.
  +## SelectMenu.MenuContext
  +SelectMenu.MenuContext is a [context object](https://reactjs.org/docs/context.html#reactcreatecontext) that exposes some helpful state values to be used via [`React.useContext`](https://reactjs.org/docs/hooks-reference.html#usecontext) in consuming applications. SelectMenu.MenuContext can only be used in components that are already wrapped in a `SelectMenu` as `SelectMenu` contains the [context provider](https://reactjs.org/docs/context.html#contextprovider).
  +
  +### Values and functions available on MenuContext
  +| Name | Type | Description |
  +| :- | :- | :- |
  +| selectedTab | string | The currently selected tab |
  +| setSelectedTab | function | Used to update the currently selected tab state |
  +| open | boolean | State for open/closed status of the menu content |
  +| setOpen | function | Used to update the `open` state |
  +| initialTab | string | Mostly used internally to pass down which tab should be set to open by default. To change this value it's best to use the `initialTab` prop on `SelectMenu` instead. |
  +
  +
  +### Example Usage
  +```jsx
  +import {SelectMenu, Button} from `@primer/components`
  +import React, {useContext} from 'react'
  +
  +const MyMenu = () => {
  + <SelectMenu>
  + <MyButton as="summary" />
  + <SelectMenu.Modal>
  + content
  + </SelectMenu.Modal>
  + </SelectMenu>
  +}
  +
  +// note that we can only use the context in components that are already wrapped by SelectMenu (and thus the Context.Provider)
  +const MyButton = () => {
  + const menuContext = useContext(SelectMenu.MenuContext);
  +
  + return (
  + <Button as="summary">{menuContext.open ? 'Open' : 'Closed'}</Button>
  + )
  +}
  +```
  ## SelectMenu.Modal
  Used to wrap the content in a `SelectMenu`.
  