update unimplemented! docs

Author: MarkMcCaskey

src/libcore/macros.rs

@@ -516,13 +516,16 @@ macro_rules! unreachable {
     });
 }
 
-/// A standardized placeholder for marking unfinished code. It panics with the
-/// message `"not yet implemented"` when executed.
+/// A standardized placeholder for marking unfinished code.
 ///
 /// This can be useful if you are prototyping and are just looking to have your
 /// code typecheck, or if you're implementing a trait that requires multiple
 /// methods, and you're only planning on using one of them.
 ///
+/// # Panics
+///
+/// This will always [panic!](macro.panic.html)
+///
 /// # Examples
 ///
 /// Here's an example of some in-progress code. We have a trait `Foo`: