`ActionSearchSource.type` is claimed to be always be `'url'`, but can be `'api'`

[DouweM]

The API docs and source code both claim that ActionSearchSource.type will always be 'url', but in fact it can be 'api' as well:

      - action:
          query: 'weather: Mexico, Mexico City'
          sources:
          - name: oai-weather
            type: api
          type: search
        id: ws_08313d91a201f39c0068c4ab69d5bc81958a568c6c8fa3b010
        status: completed
        type: web_search_call