[whatwg] Fixing undo on the Web - UndoManager and Transaction

[Ryosuke Niwa]

Hi all,

I've updated the document: http://rniwa.com/editing/undomanager.html

Summary of changes:

   - The undoscope content attribute is now ignored on an editable element
   unless it is an editing host. Sections 2.1 and 2.2 have been updated to
   reflect this change.
   - *Section 3.2 Automatic transactions* was re-organized to clarify
   details
      - It now explicitly states that the user agent is not responsible for
      fully restoring DOM state when interfered by event listeners.
      - Made highest node affecting an automatic transaction


- Ryosuke