mention payloads for non-Python kernels

they should be ignored.

docs/source/development/messaging.rst

@@ -451,6 +451,10 b" When status is 'ok', the following extra fields are present::"
 
    which appends a dictionary to the list of payloads.
 
+   The payload API is not yet stabilized,
+   and should probably not be supported by non-Python kernels at this time.
+   In such cases, the payload list should always be empty.
+
    
 When status is 'error', the following extra fields are present::