Modify NonZero documentation to reference the underlying integer type

This change updates the documentation for `NonZero` integer types to
explicitly reference the underlying integer type each `NonZero` variant
wraps, instead of  using a general "integer" term.

core/src/num/nonzero.rs

@@ -458,7 +458,15 @@ macro_rules! nonzero_integer {
         reversed = $reversed:literal,
         leading_zeros_test = $leading_zeros_test:expr,
     ) => {
-        /// An integer that is known not to equal zero.
+        #[doc = sign_dependent_expr!{
+            $signedness ?
+            if signed {
+                concat!("An [`", stringify!($Int), "`] that is known not to equal zero.")
+            }
+            if unsigned {
+                concat!("A [`", stringify!($Int), "`] that is known not to equal zero.")
+            }
+        }]
         ///
         /// This enables some memory layout optimization.
         #[doc = concat!("For example, `Option<", stringify!($Ty), ">` is the same size as `", stringify!($Int), "`:")]