Add some more hyperlinks to definitions and other specs.

DOM references:
* Document
* DocumentFragment
* ownerDocument
* ProcessingInstruction
* Text
* IndexSizeError

HTML spec references:
* HTMLElement
* addCue()

Local references:
* VTTCue
* VTTRegion
Merge pull request #65 from silviapfeiffer/add-terms

Add some more hyperlinks to definitions and other specs.