The `unreachable!` docs previously did not mention that there was a second
form, `unreachable!("message")` that could be used to specify a custom panic
message,
The docs now mention this in the same style as currently used for `unimplemented!`:
https://doc.rust-lang.org/core/macro.unimplemented.html#panics
///
/// # Panics
///
-/// This will always [`panic!`].
+/// This will always [`panic!`] because `unreachable!` is just a shorthand for `panic!` with a
+/// fixed, specific message.
+///
+/// Like `panic!`, this macro has a second form for displaying custom values.
///
/// # Examples
///
/// if 3*i < i { panic!("u32 overflow"); }
/// if x < 3*i { return i-1; }
/// }
-/// unreachable!();
+/// unreachable!("The loop should always return");
/// }
/// ```
#[macro_export]