I don't think the fix for this situation is sprinkling the docs with "is always greater than or equal to 0". I don't really think there is a doubt in someones head about whether or not a size or length function returns negative value no matter if it's unsigned int, qint64 or size_t.
I think the opposite is more reasonable - document cases where it can actually return negative value and what it means, which Qt docs are doing pretty consistently (correct me if I'm wrong).