fix(docs): mention cookies in context (#36342)

Mention that the HTTP.IncomingMessage object is augmented by `cookies` in getServerSideProps

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

docs/api-reference/data-fetching/get-server-side-props.md

@@ -31,7 +31,7 @@ You can import modules in top-level scope for use in `getServerSideProps`. Impor
 The `context` parameter is an object containing the following keys:
 
 - `params`: If this page uses a [dynamic route](/docs/routing/dynamic-routes.md), `params` contains the route parameters. If the page name is `[id].js` , then `params` will look like `{ id: ... }`.
-- `req`: [The `HTTP` IncomingMessage object](https://nodejs.org/api/http.html#http_class_http_incomingmessage).
+- `req`: [The `HTTP` IncomingMessage object](https://nodejs.org/api/http.html#http_class_http_incomingmessage), with an additional `cookies` prop, which is an object with string keys mapping to string values of cookies.
 - `res`: [The `HTTP` response object](https://nodejs.org/api/http.html#http_class_http_serverresponse).
 - `query`: An object representing the query string.
 - `preview`: `preview` is `true` if the page is in the [Preview Mode](/docs/advanced-features/preview-mode.md) and `false` otherwise.