Add more information about options in the `/api/v2/query` payload

Working through Flux HTTP examples with @rickspencer3, we found that the description for the `dialect` payload option on `/api/v2/query` doesn't really help users all that much. We need to update the descriptions in the openAPI spec to be more clear and help guide users.

One specific change we need to make is to call out (in `dialect` `description`) that to return annotated CSV in a format that can be directly written back to InfluxDB, users must include the following in their request body:

```json
{
"query": "...",
"dialect": {
"annotations": [
"group",
"datatype",
"default"
]
}
}
```

Potentially, add a request payload example that includes `group`, `datatype`, and `default` specifically for users who intend to use InfluxDB to consume the response.

##### Relevant URLs
-https://docs.influxdata.com/influxdb/v2.1/api/#operation/PostQuery