Add description for Editor onChange method

docs/api/nodes.md

@@ -429,7 +429,7 @@ Call a function, deferring normalization until after it completes.
 
 ### Instance methods
 
-#### Schema-specific actions
+#### Schema-specific methods to override
 
 ###### `isInline(element: Element)`
 
@@ -445,6 +445,9 @@ Normalize a Node according to the schema.
 
 ###### `onChange()`
 
+Called when there is a change in the editor.
+
+
 #### Core actions
 
 ###### `addMark(key: string, value: any)`