Fix wrong error messages in MultiMatchQueryParser

There is an issue with the error messages of `multi_match`:

```json
{
    "multi_match": {
        "query":    "full text search",
        "fields":   [ "title", "body" ]
    }
}
```

If I do not have any "title" or "body" fields in my mapping, it threw a parse error: `No fields specified for match_all query`. Which is wrong both ways:

- There are fields in my query
- my query is not a "match_all" :)

This PR fix this last issue and a small typo in the doc block.