docs(typenum): fix docstring warnings

Signed-off-by: Paul Spooren <mail@aparcar.org>

util/src/typenum.rs

@@ -16,6 +16,7 @@ macro_rules! typenum2const {
 
 /// Trait implemented by constant integers to facilitate conversion to constant integers
 pub trait IntoConst<T> {
+    /// The constant value after conversion
     const VALUE: T;
 }