Document the XML interfaces

How to document DBus XML: see the "XML Comments" section in the DBus API Design Guidelines.

It seems that the little documentation that there was in the didl files (see 5ff3694f) was lost when those files were removed in favor of the more up-to-date XML. Maybe we can rescue the useful docs from there?

Documentation progress:

• Accessible
• Action
• Application
• Cache
• Collection
• Component
• DeviceEventController
• DeviceEventListener
• Document
• EditableText
• Event
• Hyperlink
• Hypertext
• Image
• Registry
• Selection
• Socket
• TableCell
• Table
• Text
• Value