Documenting `cat` with an example in Apiary

I'm unsure how to document this adequately.

``` sh
$ curl -i http://localhost:5001/api/v0/cat?arg=QmeomffUNfmQy76CQGy9NdmqEnnHU9soCexBnGU3ezPHVH

HTTP/1.1 200 OK
Content-Length: 5
Transfer-Encoding: chunked
X-Stream-Output: 1

test
```

What is the output of a `cat` command? What is the mime-type of a chunked file? I don't know, and I'm having difficulty googling it.

Further, `cat` shouldn't return both `content-length` and `transfer-encoding`. See https://github.com/ipfs/go-ipfs/pull/1776 for more on that.