Insert current implementation header

src/libcore/macros.rs

@@ -456,7 +456,7 @@ macro_rules! writeln {
 ///
 /// # Panics
 ///
-/// This will always [panic!](macro.panic.html).
+/// This will always [panic!](macro.panic.html)
 ///
 /// # Examples
 ///

src/libstd/macros.rs

@@ -24,6 +24,8 @@
 /// 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 return with code `101`.
 ///
 /// # Examples