Use unsafe in the unsafe attribute examples.

This will be required in 2024.

src/abi.md

@@ -69,6 +69,11 @@ object file, similar to the [`used` attribute](#the-used-attribute).
 This attribute is unsafe as an unmangled symbol may collide with another symbol
 with the same name (or with a well-known symbol), leading to undefined behavior.
 
+```rust
+#[unsafe(no_mangle)]
+extern "C" fn foo() {}
+```
+
 ## The `link_section` attribute
 
 The *`link_section` attribute* specifies the section of the object file that a
@@ -80,8 +85,8 @@ of memory not expecting them, such as mutable data into read-only areas.
 
 <!-- no_run: don't link. The format of the section name is platform-specific. -->
 ```rust,no_run
-#[no_mangle]
-#[link_section = ".example_section"]
+#[unsafe(no_mangle)]
+#[unsafe(link_section = ".example_section")]
 pub static VAR1: u32 = 1;
 ```
 
@@ -96,7 +101,7 @@ symbol with the same name (or with a well-known symbol), leading to undefined
 behavior.
 
 ```rust
-#[export_name = "exported_symbol_name"]
+#[unsafe(export_name = "exported_symbol_name")]
 pub fn name_in_rust() { }
 ```