Be more explicit on the framed success response
It's the same format as the other responses, but it wasn't obvious
before that the first 2 bytes of the payload were a response code.
They'll probably always be set to 0 for success.
@@ -614,7 +614,7 @@ The following types are defined:
Type | 2 byte value | Description
---------------|--------------|------------
-Success | "OK" | The command was executed successfully. The payload contains the result.
+Success | "OK" | The command was executed successfully. The payload is a 2 bytes result code (currently 0 for success) followed by an optional textual message.
Error | "ER" | A failure occurred. The payload is a 2 byte error code (future use) followed by a textual error message.
Warning | "WN" | A warning occurred. The payload is a 2 byte error code (future use) followed by a textual error message.
Progress | "PR" | The next two bytes are the progress (0-100) as a big endian integer.
