all new fields are backward-compatible

not just on cells, outputs, metadata

docs/source/notebook/nbformat.rst

@@ -291,8 +291,8 b' regardless of format.'
       "source" : ["some nbformat mime-type data"]
     }
 
-Backwared-compatible changes
-============================
+Backward-compatible changes
+===========================
 
 The notebook format is an evolving format. When backward-compatible changes are made,
 the notebook format minor version is incremented. When backward-incompatible changes are made,
@@ -300,7 +300,7 b' the major version is incremented.'
 
 As of nbformat 4.x, backward-compatible changes include:
 
-- new fields in any cell or output or metadata
+- new fields in any dictionary (notebook, cell, output, metadata, etc.)
 - new cell types
 - new output types