Add comment on largest input and explain safety assumption

Author: brokemillenial

library/core/src/time.rs

@@ -311,7 +311,8 @@ impl Duration {
     ///
     /// # Panics
     ///
-    /// Panics if the given number of weeks overflows the `Duration` size.
+    /// Panics if the given number of nanoseconds is greater than what Duration can handle, 
+    /// which is `(u64::MAX * NANOS_PER_SEC) + NANOS_PER_SEC - 1`
     /// Use this function if you need to specify time greater than what can fit in u64
     /// (around 584 years).
     ///
@@ -333,7 +334,7 @@ impl Duration {
             panic!("overflow in duration in Duration::from_nanos_u128");
         }
         let subsec_nanos = (nanos % NANOS_PER_SEC) as u32;
-        // SAFETY: x % 1_000_000_000 < 1_000_000_000
+        // SAFETY: x % 1_000_000_000 < 1_000_000_000 also, subsec_nanos >= 0 since u128 >=0 and u32 >=0
         let subsec_nanos = unsafe { Nanoseconds::new_unchecked(subsec_nanos) };
 
         Duration { secs: secs as u64, nanos: subsec_nanos }