Fix some inconsistencies in messages api ref

I512bfc97a1c3d4a508daf580d99fefe2a9ae4e13 tries to add response
schema validation for messages interfaces, and there are some
inconsistencies in api ref:
1. delete message success code is 204, not 202
2. id is of the format uuid, which means string, not integer.

Change-Id: I13280bf158f541ed0514930274df8c03a0f81c4c

api-ref/source/v3/messages.inc

@@ -20,7 +20,7 @@ Response codes
 
 .. rest_status_code:: success ../status.yaml
 
-   - 202
+   - 204
 
 .. rest_status_code:: error ../status.yaml
 

api-ref/source/v3/parameters.yaml

@@ -1363,10 +1363,10 @@ id_7:
   type: integer
 id_8:
   description: |
-    The ID for the message.
+    The UUID for the message.
   in: body
   required: true
-  type: integer
+  type: string
 image_id:
   description: |
     The uuid for the new image.