docs: improve m.request return value description

In the m.request return value description, add a line informing that
error status codes cause the promise to reject.

docs/request.md

@@ -61,7 +61,7 @@ Argument                  | Type                              | Required | Descr
 `options.deserialize`     | `any = Function(any)`          | No       | A deserialization method to be applied to the `xhr.response` or normalized `xhr.responseText`. Defaults to the [identity function](https://en.wikipedia.org/wiki/Identity_function). If `extract` is defined, `deserialize` will be skipped.
 `options.extract`         | `any = Function(xhr, options)`    | No       | A hook to specify how the XMLHttpRequest response should be read. Useful for processing response data, reading headers and cookies. By default this is a function that returns `options.deserialize(parsedResponse)`, throwing an exception when the server response status code indicates an error or when the response is syntactically invalid. If a custom `extract` callback is provided, the `xhr` parameter is the XMLHttpRequest instance used for the request, and `options` is the object that was passed to the `m.request` call. Additionally, `deserialize` will be skipped and the value returned from the extract callback will be left as-is when the promise resolves.
 `options.background`      | `Boolean`                         | No       | If `false`, redraws mounted components upon completion of the request. If `true`, it does not. Defaults to `false`.
-**returns**               | `Promise`                         |          | A promise that resolves to the response data, after it has been piped through the `extract`, `deserialize` and `type` methods
+**returns**               | `Promise`                         |          | A promise that resolves to the response data, after it has been piped through the `extract`, `deserialize` and `type` methods. If the response status code indicates an error, the promise rejects, but this can be prevented by setting the `extract` option. 
 
 `promise = m.request(url, options)`