Docs reference a `putChannel` mutation but none such exists

[dperl-dls]

https://diamondlightsource.github.io/coniql/main/how-to/run-a-server.html#ca-plugin says that one may put to a pv using a query of the format:

mutation {
  putChannel(id: "ca://pc0105:longout", value: "45") {
    value {
      string(units: true)
    }
  }
}

however, the only mutation which exists is putChannels which takes a list of PVs and values