Is iCloud Document Sync "delta sync" or "full file sync"? I don't mean iWork docs but for example personal docs in GoodReader iCloud folder and perhaps Mac OS ~/Library/Mobile Documents.

Is there any official documentation about it?
Delta. It splits this intelligently into chunks and only updates the relevant chunks using Rabin fingerprinting.

You can see this in Apple's WWDC iCloud presentation, and 
slides.

A pretty good description of this chunking can be seen in Siracusa's Lion review, however the review describes this process in terms of document versioning, but the fundamentals are the same.