fix outdated payload part of messaging spec

docs/source/development/messaging.txt

@@ -324,22 +324,17 b' Message type: ``execute_reply``::'
 When status is 'ok', the following extra fields are present::
 
     {
-      # The execution payload is a dict with string keys that may have been
+      # 'payload' will be a list of payload dicts.
+      # Each execution payload is a dict with string keys that may have been
       # produced by the code being executed.  It is retrieved by the kernel at
       # the end of the execution and sent back to the front end, which can take
       # action on it as needed.  See main text for further details.
-      'payload' : dict,
+      'payload' : list(dict),
 
       # Results for the user_variables and user_expressions.
       'user_variables' : dict,
       'user_expressions' : dict,
-
-      # The kernel will often transform the input provided to it.  If the
-      # '---->' transform had been applied, this is filled, otherwise it's the
-      # empty string.  So transformations like magics don't appear here, only
-      # autocall ones.
-      'transformed_code' : str,
-      }
+    }
 
 .. admonition:: Execution payloads
     
@@ -347,13 +342,12 b" When status is 'ok', the following extra fields are present::"
    given set of code, which normally is just displayed on the pyout stream
    through the PUB socket.  The idea of a payload is to allow special types of
    code, typically magics, to populate a data container in the IPython kernel
-   that will be shipped back to the caller via this channel.  The kernel will
-   have an API for this, probably something along the lines of::
+   that will be shipped back to the caller via this channel.  The kernel
+   has an API for this in the PayloadManager::
 
-       ip.exec_payload_add(key, value)
+       ip.payload_manager.write_payload(payload_dict)
 
-   though this API is still in the design stages.  The data returned in this
-   payload will allow frontends to present special views of what just happened.
+   which appends a dictionary to the list of payloads.
 
    
 When status is 'error', the following extra fields are present::