std: Apply missing_unsafe_on_extern

library/std/src/keyword_docs.rs

@@ -1428,7 +1428,7 @@ mod self_upper_keyword {}
 ///
 /// ```rust,no_run
 /// # #![allow(dead_code)]
-/// extern "C" {
+/// unsafe extern "C" {
 ///     static mut ERROR_MESSAGE: *mut std::os::raw::c_char;
 /// }
 /// ```
@@ -1925,7 +1925,7 @@ mod type_keyword {}
 ///
 /// unsafe fn unsafe_fn() {}
 ///
-/// extern "C" {
+/// unsafe extern "C" {
 ///     fn unsafe_extern_fn();
 ///     static BAR: *mut u32;
 /// }