Cannot search Cores on serial part
I'm requesting `POST /v4/cores/query` with the following request body to get a filtered output of Boosters (without the Merlins):

```json
{
  "options": {
    "sort": {
      "serial": "desc"
    }
  },
  "query": {
    "$text": {
      "$search": "B"
    }
  }
}
```

I'm receiving a **400**-error: `index required for $text query`

Aren't cores searchable by all text indices? Is this the wrong structure? 
@jakewmeyer would it be possible to add a partial string match example in the docs?