Update Changes.txt for include CompositionEvent change.
Duplicate the document:
  Overview.html for primary IME API (UI events)
  Overview2.html for secondary IME API (Composition)
Rename Overview2 to Annex.
Cut off CompositionEvent and rewrite documentation.
Add a note about the status of this document.
Remove setCaretRectangle/{enable,disable}EditingEvents
Fix reference to 'locale' and 'composition' in Overview.html
Remove 'IME reference' background section in Annex.
Add a note for 'locale'
Fix wording of Annex's note.
Update Changes.txt