I totally disagree. Git is a great way to store documentation. Also „Git“ and „Wiki“ are not opposites. You can have a wiki AND store content in git at the same time.
Great in which sense?
I would argue that git is easy because you are already familiar with it, but it doesn't make shared documentation simple.
How could he do that?
Do you think that Linus Torvalds had a use case similar to Wikipedia in mind when he designed git?
Sure you could build Wikipedia on top of git, it would just be shitload of unnecessary suffering compared to the hipster technology called PHP+MySQL.