Fix comments for unsigned non-zero `checked_add`, `saturating_add`
While looking at #118313, I happened to notice that two of the expanded comments appear to be slightly inaccurate.
For these two methods, `other` is an ordinary unsigned integer, so it can be zero.
Since the sum of non-zero and zero is always non-zero, the safety argument holds even when `other` is zero.
4f3ee302b7