Rollup merge of #88712 - jhpratt:fix-int_rounding-docs, r=joshtriplett

Fix docs for `uX::checked_next_multiple_of`

Thanks to `@photino` for noticing this [here](https://github.com/rust-lang/rust/issues/88581#issuecomment-913982246).

r? `@joshtriplett`

`@rustbot` label: +A-docs +A-waiting-on-review

library/core/src/num/uint_macros.rs

@@ -1924,7 +1924,8 @@ macro_rules! uint_impl {
         }
 
         /// Calculates the smallest value greater than or equal to `self` that
-        /// is a multiple of `rhs`. If `rhs` is negative,
+        /// is a multiple of `rhs`. Returns `None` is `rhs` is zero or the
+        /// operation would result in overflow.
         ///
         /// # Examples
         ///