Clean up core macros documentation

* Switch a couple links over to intra-doc links
* Clean up some formatting/typography

library/core/src/macros/mod.rs

@@ -517,15 +517,15 @@ macro_rules! unreachable {
 /// This allows your code to type-check, which is useful if you are prototyping or
 /// implementing a trait that requires multiple methods which you don't plan of using all of.
 ///
-/// The difference between `unimplemented!` and [`todo!`](macro.todo.html) is that while `todo!`
+/// The difference between `unimplemented!` and [`todo!`] is that while `todo!`
 /// conveys an intent of implementing the functionality later and the message is "not yet
 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
 /// Also some IDEs will mark `todo!`s.
 ///
 /// # Panics
 ///
-/// This will always [panic!](macro.panic.html) because `unimplemented!` is just a
-/// shorthand for `panic!` with a fixed, specific message.
+/// This will always [`panic!`] because `unimplemented!` is just a shorthand for `panic!` with a
+/// fixed, specific message.
 ///
 /// Like `panic!`, this macro has a second form for displaying custom values.
 ///
@@ -602,7 +602,7 @@ macro_rules! unimplemented {
 ///
 /// # Panics
 ///
-/// This will always [panic!](macro.panic.html)
+/// This will always [`panic!`].
 ///
 /// # Examples
 ///