Correct documentation for PointerLike about automatic implementation.

Since <rust-lang#133226>, it is no longer
automatically implemented, but must be manually implemented.

Author: kpreid

library/core/src/marker.rs

@@ -972,8 +972,8 @@ pub trait Tuple {}
 
 /// A marker for pointer-like types.
 ///
-/// All types that have the same size and alignment as a `usize` or
-/// `*const ()` automatically implement this trait.
+/// This trait may only be implemented for types that have the same size and alignment as a
+/// [`usize`] or [`*const ()`](pointer).
 #[unstable(feature = "pointer_like_trait", issue = "none")]
 #[lang = "pointer_like"]
 #[diagnostic::on_unimplemented(