Rollup merge of rust-lang#42597 - mark-buer:park_timeout_example_fix, r=alexcrichton

Capture elapsed duration in Thread::park_timeout example

`beginning_park.elapsed()` might return a larger value within the loop as compared to that checked in the loop conditional.
Since `Duration` arithmetic is checked, hitting such an edge case will cause a panic.

Author: frewsxcv

src/libstd/thread/mod.rs

@@ -787,12 +787,16 @@ pub fn park_timeout_ms(ms: u32) {
 ///
 /// let timeout = Duration::from_secs(2);
 /// let beginning_park = Instant::now();
-/// park_timeout(timeout);
 ///
-/// while beginning_park.elapsed() < timeout {
-///     println!("restarting park_timeout after {:?}", beginning_park.elapsed());
-///     let timeout = timeout - beginning_park.elapsed();
-///     park_timeout(timeout);
+/// let mut timeout_remaining = timeout;
+/// loop {
+///     park_timeout(timeout_remaining);
+///     let elapsed = beginning_park.elapsed();
+///     if elapsed >= timeout {
+///         break;
+///     }
+///     println!("restarting park_timeout after {:?}", elapsed);
+///     timeout_remaining = timeout - elapsed;
 /// }
 /// ```
 ///