improve doc for json_object_to_json_string()

The function returns a pointer, after all converting and stuff, it's this in the end:

```C
return jso->_pb->buf;
```

This is an internal buffer handled by the json-c library itself. I suppose the size is reallocated on purpose. Doc update clarifies, the user should not free this and use strdup() in concurrent environments. I hope I understood this correctly.