Adjust how it looks in the docs

Author: Darksonn

library/core/src/pin.rs

@@ -1708,6 +1708,7 @@ mod hidden {
 }
 
 #[stable(feature = "pin", since = "1.33.0")]
+#[cfg(not(doc))]
 impl<Ptr> DerefMut for Pin<Ptr>
 where
     Ptr: Deref,
@@ -1720,6 +1721,20 @@ where
     }
 }
 
+// This impl is only used during documentation, but it is fully equivalent to the above impl with
+// the exception of how the orphan rule treats it.
+#[stable(feature = "pin", since = "1.33.0")]
+#[cfg(doc)]
+impl<Ptr> DerefMut for Pin<Ptr>
+where
+    Ptr: DerefMut,
+    Ptr::Target: Unpin,
+{
+    fn deref_mut(&mut self) -> &mut Ptr::Target {
+        Pin::get_mut(Pin::as_mut(self))
+    }
+}
+
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<Ptr: DerefPure> DerefPure for Pin<Ptr> {}