Document workaround in docs

Author: Darksonn

library/core/src/pin.rs

@@ -1755,13 +1755,18 @@ where
     }
 }
 
+/// Restricted to `Unpin` types, as it's not safe to obtain a mutable reference to a pinned value.
+///
+/// Note that because `Pin` is fundamental, it would normally be possible for crates to implement
+/// `DerefMut` for `Pin<T>` when `T` is a local type and not covered by this impl block. However,
+/// this impl block is written in a special way to prevent that for soundness reasons.
 #[stable(feature = "pin", since = "1.33.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
 #[cfg(doc)]
 impl<Ptr> const DerefMut for Pin<Ptr>
 where
     Ptr: [const] DerefMut,
-    Ptr::Target: Unpin,
+    <Ptr as Deref>::Target: Unpin,
 {
     fn deref_mut(&mut self) -> &mut Ptr::Target {
         Pin::get_mut(Pin::as_mut(self))