Add another example for uN::carrying_mul

The prose talked about doing this, so might as well add a simple code example of it too.

library/core/src/num/mod.rs

@@ -113,6 +113,9 @@ macro_rules! widening_impl {
         /// This returns the low-order (wrapping) bits and the high-order (overflow) bits
         /// of the result as two separate values, in that order.
         ///
+        /// If you also need to add a carry to the wide result, then you want
+        /// [`Self::carrying_mul`] instead.
+        ///
         /// # Examples
         ///
         /// Basic usage:
@@ -148,6 +151,8 @@ macro_rules! widening_impl {
         /// additional amount of overflow. This allows for chaining together multiple
         /// multiplications to create "big integers" which represent larger values.
         ///
+        /// If you don't need the `carry`, then you can use [`Self::widening_mul`] instead.
+        ///
         /// # Examples
         ///
         /// Basic usage:
@@ -167,6 +172,31 @@ macro_rules! widening_impl {
         )]
         /// ```
         ///
+        /// This is the core operation needed for scalar multiplication when
+        /// implementing it for wider-than-native types.
+        ///
+        /// ```
+        /// #![feature(bigint_helper_methods)]
+        /// fn scalar_mul_eq(little_endian_digits: &mut Vec<u16>, multiplicand: u16) {
+        ///     let mut carry = 0;
+        ///     for d in little_endian_digits.iter_mut() {
+        ///         (*d, carry) = d.carrying_mul(multiplicand, carry);
+        ///     }
+        ///     if carry != 0 {
+        ///         little_endian_digits.push(carry);
+        ///     }
+        /// }
+        ///
+        /// let mut v = vec![10, 20];
+        /// scalar_mul_eq(&mut v, 3);
+        /// assert_eq!(v, [30, 60]);
+        ///
+        /// assert_eq!(0x87654321_u64 * 0xFEED, 0x86D3D159E38D);
+        /// let mut v = vec![0x4321, 0x8765];
+        /// scalar_mul_eq(&mut v, 0xFEED);
+        /// assert_eq!(v, [0xE38D, 0xD159, 0x86D3]);
+        /// ```
+        ///
         /// If `carry` is zero, this is similar to [`overflowing_mul`](Self::overflowing_mul),
         /// except that it gives the value of the overflow instead of just whether one happened:
         ///