Rollup merge of #66114 - golddranks:improve_thread_docs, r=Centril

Improve std:🧵:Result documentation

Thanks to @dtolnay for pointing out the different premise of the contents of the `Err` variant in `std:🧵:Result` WRT normal error handling.

src/libstd/thread/mod.rs

@@ -1271,6 +1271,18 @@ impl fmt::Debug for Thread {
 ///
 /// Indicates the manner in which a thread exited.
 ///
+/// The value contained in the `Result::Err` variant
+/// is the value the thread panicked with;
+/// that is, the argument the `panic!` macro was called with.
+/// Unlike with normal errors, this value doesn't implement
+/// 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
+/// 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.
+///
 /// A thread that completes without panicking is considered to exit successfully.
 ///
 /// # Examples