Update extern documentation for Rust 2024

Author: t5kd

library/std/src/keyword_docs.rs

@@ -381,11 +381,13 @@ mod enum_keyword {}
 /// lazy_static;`. The other use is in foreign function interfaces (FFI).
 ///
 /// `extern` is used in two different contexts within FFI. The first is in the form of external
-/// blocks, for declaring function interfaces that Rust code can call foreign code by.
+/// blocks, for declaring function interfaces that Rust code can call foreign code by. Starting
+/// with Rust 2024, external blocks must be marked as `unsafe`. In earlier editions, use of
+/// the `unsafe` keyword remains optional.
 ///
 /// ```rust ignore
 /// #[link(name = "my_c_library")]
-/// extern "C" {
+/// unsafe extern "C" {
 ///     fn my_c_function(x: i32) -> bool;
 /// }
 /// ```