Possibly because the Go "authorities" are now trying to attract the kind of programmers who need "trigger warnings" and "safe spaces", for some reason. Just look at the new community CoC. I apologize if this constitutes a micro- or nano-aggression.

More seriously, it's probably just a frequent mistake and they try to educate people proactively to help code quality in the long run.

Edit: the non-atomic map updates are mentioned in the FAQ, but you're right, the documentation could be improved in that regard (possibly triggering many criticisms of the language from people who've never used it, just browsed the docs for reasons not to use it).