I don't mean iWork docs but for example personal docs in GoodReader
iCloud folder and perhaps Mac OS
Is there any official documentation about it?
Delta. It splits this intelligently into chunks and only updates the relevant chunks using Rabin fingerprinting.
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.
I think that it is full file sync. The delta aspect that you are wondering about would be with Versions (or Time Machine), and that is a separate set of functionality beyond the scope of iCloud's Documents.