note about mime-bundle in nbformat doc

docs/source/notebook/nbformat.rst

@@ -158,11 +158,13 b' stream output'
 display_data
 ************
 
-Rich display messages (as created by ``display_data`` messages)
-contain data keyed by mime-type. All mime-type data should
+Rich display outputs, as created by ``display_data`` messages,
+contain data keyed by mime-type. This is often called a mime-bundle,
+and shows up in various locations in the notebook format and message spec.
 The metadata of these messages may be keyed by mime-type as well.
 
 
+
 .. sourcecode:: python
 
     {