Suggest less bug-prone construction of Duration in docs

Author: BenWiederhake

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.subsec_nanos())`
+    /// if you cannot copy/clone the Duration directly.
+    ///
     /// # Examples
     ///
     /// ```