Currently it's redundant and not easy to read.
```
{"message":"JSON object requested, multiple (or no) rows returned",
"details":"The result contains 2 rows"}
```
Now:
```
{"message":"Cannot coerce the result to a single JSON object",
"details":"The result contains an array of 0 objects"}
``
Also correct docs which had a wrong error code.
* Clarify vnd.pgrst.object+json is also Content-Type
The previous docs explained how the `Accept` header could be set to
`application/vnd.pgrst.object+json`. This change indicates the resulting
response `Content-Type` begins likewise.
While robust clients will correctly identify that
`application/vnd.pgrst.object+json` data are json, many others which
naively check the `Content-Type` is `application/json` will fail to
recognize what should be a valid `Content-Type`.
