Recommend panic::resume_unwind instead of panicking.

Fixes https://github.com/rust-lang/rust/issues/79950.

library/std/src/thread/mod.rs

@@ -1186,32 +1186,37 @@ impl fmt::Debug for Thread {
 /// the [`Error`](crate::error::Error) trait.
 ///
 /// Thus, a sensible way to handle a thread panic is to either:
-/// 1. `unwrap` the `Result<T>`, propagating the panic
+///
+/// 1. propagate the panic with [`std::panic::resume_unwind`]
 /// 2. or in case the thread is intended to be a subsystem boundary
 /// that is supposed to isolate system-level failures,
-/// match on the `Err` variant and handle the panic in an appropriate way.
+/// match on the `Err` variant and handle the panic in an appropriate way
 ///
 /// A thread that completes without panicking is considered to exit successfully.
 ///
 /// # Examples
 ///
+/// Matching on the result of a joined thread:
+///
 /// ```no_run
-/// use std::thread;
-/// use std::fs;
+/// use std::{fs, thread, panic};
 ///
 /// fn copy_in_thread() -> thread::Result<()> {
-///     thread::spawn(move || { fs::copy("foo.txt", "bar.txt").unwrap(); }).join()
+///     thread::spawn(|| {
+///         fs::copy("foo.txt", "bar.txt").unwrap();
+///     }).join()
 /// }
 ///
 /// fn main() {
 ///     match copy_in_thread() {
-///         Ok(_) => println!("this is fine"),
-///         Err(_) => println!("thread panicked"),
+///         Ok(_) => println!("copy succeeded"),
+///         Err(e) => panic::resume_unwind(e),
 ///     }
 /// }
 /// ```
 ///
 /// [`Result`]: crate::result::Result
+/// [`std::panic::resume_unwind`]: crate::panic::resume_unwind
 #[stable(feature = "rust1", since = "1.0.0")]
 pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;