docs: Upate React example usage with MSW to mention that fetch will require polyfill (#1418)

* Update example-intro.mdx

Updates the example notes re: MSW to mention that jsdom doesn't include fetch and will require a polyfill

* Fix link

* Update example-intro.mdx

* Update example-intro.mdx

* Update example-intro.mdx

Author: dwjohnston

docs/react-testing-library/example-intro.mdx

@@ -74,6 +74,12 @@ declaratively mock API communication in your tests instead of stubbing
 
 :::
 
+:::note 
+
+Our example here uses axios to make its API calls. If your application uses [`fetch()`](https://developer.mozilla.org/en-US/docs/Web/API/Window/fetch) to make its API calls, then be aware that by default JSDOM does not include fetch. If you are using vitest as your test runner, it will be included for you. For jest you may wish to manually polyfill `fetch()` or use the [jest-fixed-jsdom](https://github.com/mswjs/jest-fixed-jsdom) environment which includes fetch. 
+
+:::
+
 ```jsx title="__tests__/fetch.test.jsx"
 import React from 'react'
 import {http, HttpResponse} from 'msw'