The documented JSON output for `/api/v0/dag/import` is incorrect

The current documentation describes `/api/v0/dag/import` as returning a JSON body with the following structure:

```json
{
"Root": {
"Cid": {
"/": ""
},
"PinErrorMsg": ""
},
"Stats": {
"BlockBytesCount": "",
"BlockCount": ""
}
}
```

What is actually returned is a JSONL body with the following structure:

```jsonl
{"Root":{"Cid":{"/":""},"PinErrorMsg":"string"}}
{"Stats":{"BlockCount":,"BlockBytesCount":}}
```

Note that the `Stats` "record" is only returned if the `stats=true` argument is provided.