message: docs: describe format for Buffers
diff: @@ -357,3 +357,11 @@ ValueId:
  ```
  Data: `args.parameters`
  +
  +## Buffer payloads
  +
  +When an MQTT message contains a value of type `Buffer`, such as an Api call argument or return value, the buffer's content is represented as a JSON object of this form:
  +
  +```json
  +{ type: "Buffer", data: [1, 2, 3] }
  +```
  