Bug 1053940 - update docker-worker docs for encrypted variables

How about calling it "Raw message example".

And show that it is provided as:
```js
task: {
  ...
  payload: {
    ...
    encryptEnv: ["<base64(encrypted(raw-message))>"]
  }
}
```

Where `encrypted(raw-message)` is gpg encrypted using the public key. We should probably also say where the public key is stored. I suggest that we upload it to `reference.taskcluster.net/docker-worker/v1/key.pub`.
(reference.taskcluster.net is an S3 bucket with a CNAME).