Chore: Update Swagger API docs (#608)

swagger/definitions/resource/message.yml

@@ -2,15 +2,23 @@ type: object
 properties:
   content:
     type: string
-  inbox_id:
-    type: number
-  conversation_id:
-    type: number
+    description: The text content of the message
   message_type:
     type: string
+    enum: ["incoming", "outgoing", "activity", "template"]
+    description: The type of the message
   created_at:
     type: integer
+    description: The time at which message was created
   private:
     type: boolean
+    description: The flags which shows whether the message is private or not
   attachment:
     type: object
+    description: The file object attached to the image
+  sender:
+    type: object
+    description: User/Agent/AgentBot object
+  conversation_id:
+    type: number
+    description: ID of the conversation