Rollup merge of rust-lang#42670 - dns2utf8:panic_return_code, r=steveklabnik

Add hint about the return code of panic!

I hope the link works on all cases, since the `unreachable` doc is copied to `std::` as well.

Author: Mark-Simulacrum

src/libcore/macros.rs

@@ -462,7 +462,7 @@ macro_rules! writeln {
 ///
 /// # Panics
 ///
-/// This will always panic.
+/// This will always [panic!](macro.panic.html)
 ///
 /// # Examples
 ///

src/libstd/macros.rs

@@ -24,6 +24,11 @@
 /// The multi-argument form of this macro panics with a string and has the
 /// `format!` syntax for building a string.
 ///
+/// # Current implementation
+///
+/// If the main thread panics it will terminate all your threads and end your
+/// program with code `101`.
+///
 /// # Examples
 ///
 /// ```should_panic