[labeled-response] Clarify self echo-message rationale. Addresses #379

Author: jwheare

extensions/labeled-response.md

@@ -71,7 +71,7 @@ If no response is required, an empty batch MUST be sent.
 
 If `batch` has not been enabled, the tag MAY be included on only one of the messages in the response.
 
-When a client sends a message to itself, the server MUST NOT include the label tag, except for any acknowledgment sent with the [`echo-message`](/specs/extensions/echo-message-3.2.html) mechanism. This is because the delivered message is not a response, but a side effect.
+When a client sends a message to itself, the server MUST NOT include the label tag, except for any acknowledgment sent with the [`echo-message`](/specs/extensions/echo-message-3.2.html) mechanism. This allows clients to differentiate between the echoed message response, and the delivered message.
 
 #### Tag value