Update docs to properly describe delete with pop

This patch updates the documentation for the Zaqar API to properly
describe when a delete is invoked with the pop query parameter.

Change-Id: Id586797eb4707f5db9d751c80035ba07270627c3
Closes-Bug: 1768157

api-ref/source/messages.inc

@@ -249,6 +249,7 @@ Response codes
 
 .. rest_status_code:: success status.yaml
 
+   - 200
    - 204
 
 
@@ -263,6 +264,8 @@ Response codes
 Request Parameters
 ------------------
 
+This operation does not accept a request body.
+
 .. rest_parameters:: parameters.yaml
 
   - queue_name: queue_name
@@ -270,8 +273,14 @@ Request Parameters
   - pop: pop
 
 
-This operation does not accept a request body and does not return a response
-body.
+Response Example
+----------------
+
+This operation only returns a response body when the ``pop`` query parameter
+is used.
+
+.. literalinclude:: samples/messages-delete-bypop-response.json
+   :language: javascript
 
 
 Get A Specific Message

api-ref/source/samples/messages-delete-bypop-response.json

@@ -0,0 +1,16 @@
+{
+  "messages": [
+    {
+      "body": {
+        "current_bytes": "0",
+        "event": "BackupProgress",
+        "total_bytes": "99614720"
+      },
+      "age": 443,
+      "claim_count": 1,
+      "claim_id": "51db7067821e727dc24df754",
+      "id": "578f0055508f153f256f717f",
+      "ttl": 3600
+    }
+  ]
+}