Suggest less bug-prone construction of Duration in docs

library/core/src/time.rs

@@ -268,6 +268,11 @@ impl Duration {
 
     /// Creates a new `Duration` from the specified number of nanoseconds.
     ///
+    /// Note: Using this on the return value of `as_nanos()` might cause unexpected
+    /// behavior: `as_nanos()` returns a u128, and can return values that do not
+    /// fit in u64, e.g. 585 years. Instead, consider using the pattern
+    /// `Duration::new(d.as_secs(), d.subsecs_nanos())`.
+    ///
     /// # Examples
     ///
     /// ```