Rollup merge of rust-lang#44206 - MarkMcCaskey:master, r=steveklabnik

update unimplemented! docs

For rust-lang#42628 (updating docs from changes from rust-lang#42155).

Initial changes made to make `unimplemented!` doc comments look more like `unreachable!` and remove statement about the panic message.

r? @steveklabnik

src/libcore/macros.rs

@@ -531,15 +531,13 @@ macro_rules! unreachable {
 
 /// A standardized placeholder for marking unfinished code.
 ///
-/// It panics with the message `"not yet implemented"` when executed.
-///
 /// 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 macro always panics.
+/// This will always [panic!](macro.panic.html)
 ///
 /// # Examples
 ///