[security] Server-side rendering docs allow XSS attack

[danilobuerger]

https://www.apollographql.com/docs/react/features/server-side-rendering.html#store-rehydration

Don't use this technique!

<script>
  window.__APOLLO_STATE__ = client.extract();
</script>

allows for XSS as potential user input is not sanitised. See for example:
https://redux.js.org/recipes/server-rendering#security-considerations

[abernix]

The example in the documentation has been updated to include JSON.stringify after #4287, and I've updated the original comment here to ensure that this technique is not used.