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

[Ryosuke Niwa]

Hi all,

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

Summary of changes:

   - "transaction" is renamed to "DOM transaction"
   - The value of isAutomatic is read immediately before the transaction is
   applied
   - Swapped the definition of clearUndo and clearRedo in the non-normative
   section to match the rest of the spec
   - transact() no longer attempts to merge transactions when there are no
   transactions to merge with
   - Incorporated Aryeh's feedback
   - DOM state now include IDL properties and their values

Best,
Ryosuke Niwa
Software Engineer
Google Inc.