I'd argue that is a significant contribution. How many open-source projects are well-documented? Very few!
Granted, I'd personally prefer to be able to view a candidate's coding style, but I would certainly not ignore a candidate who contributed to documentation on a project. I'd just make the tech interview a little harder ;)
Documentation won't necessarily let you see someone's coding quality, but like technical blogging it will give you an insight into the level of technical capability the person has and their ability to speak clearly on technical subjects. All of which are very critical skills on the job.
