I agree that it's a somewhat hostile way to invite people to a project, but 1) I highly doubt that dropping a cheerful invite will have any convincing power other than someone replying "yeah, I should do that" and then promptly forgetting it, and 2) some scolding was needed. :)

Your second point is a good one, but the developers are in a better position to write actual features and fix bugs than anyone else, and since documentation is more public-facing than source code, it's actually a better choice if someone wants to contribute anything at all, so the primary developers can spend their time more wisely. I certainly know the "phenomenally faster if I'd just written it in the first place" effect well, which is why I recommend to contributers to not do drive-by contributions but spend a good week writing notes while they use the library, and then convert to high-quality documentation, effectively adopting a section like you would with a community garden. Quality over quantity. And if the documentation attempt is low quality, it's still a much nicer way to prod the developer to write some documentation, so he/she can use it to define their quality standards or as a possible outline.