Improve documentation regarding response.headers

The Redaxios documentation refers us to the Axios documentation but that's not correct for some features. In the case of `response.headers` if we were to follow the Axios documentation to retrieve say `Content-Disposition` it would look like this:

```js
response.headers['Content-Disposition']
```

However, redaxios has forwarded the fetch api response interface that is like so:

```js
response.headers.get('Content-Disposition')
```

It would be nice to have better documentation that points out the differences between Axios and Redaxios and further highlights that redaxios is definitely not a drop-in replacement for axios.

Start with the Redaxios documentation and compare its Axios references with the Fetch-style response.headers API described in the issue. Document that Content-Disposition is retrieved with response.headers.get('Content-Disposition'), explain the relevant Axios differences, and make clear that Redaxios is not a drop-in Axios replacement.

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