fix(core): update doctest; address pr(rust-lang#139243) comments

Author: actuallylost

library/core/src/time.rs

@@ -308,31 +308,32 @@ impl Duration {
         Duration { secs, nanos: subsec_nanos }
     }
 
-    /// Creates a new Duration from the specified number of nanoseconds.
+    /// Creates a new `Duration` from the specified number of nanoseconds.
     ///
     /// # Panics
     ///
-    /// 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).
+    /// Panics if the given number of nanoseconds is greater than [`Duration::MAX`].
     ///
     /// # Examples
     ///
     /// ```
     /// #![feature(duration_from_nanos_u128)]
     /// use std::time::Duration;
-    /// let time_in_nanos = 2u128.pow(64);
-    /// let duration = Duration::from_nanos_u128(time_in_nanos);
+    ///
+    /// let duration = Duration::from_nanos_u128(1_000_000_321);
+    ///
+    /// assert_eq!(1, duration.as_secs());
+    /// assert_eq!(321, duration.subsec_nanos());
     /// ```
     #[unstable(feature = "duration_from_nanos_u128", issue = "139201")]
     #[must_use]
     #[inline]
+    #[track_caller]
     pub const fn from_nanos_u128(nanos: u128) -> Duration {
         const NANOS_PER_SEC: u128 = self::NANOS_PER_SEC as u128;
         let secs: u128 = nanos / NANOS_PER_SEC;
         if secs > u64::MAX as u128 {
-            panic!("overflow in duration in Duration::from_nanos_u128");
+            panic!("overflow in `Duration::from_nanos_u128`");
         }
         let subsec_nanos = (nanos % NANOS_PER_SEC) as u32;
         // SAFETY: x % 1_000_000_000 < 1_000_000_000 also, subsec_nanos >= 0 since u128 >=0 and u32 >=0