link Unpin to pin module documentation

src/libcore/marker.rs

@@ -611,6 +611,7 @@ unsafe impl<'a, T: ?Sized> Freeze for &'a mut T {}
 /// Instead it can be used to prevent moves through the type system,
 /// by controlling the behavior of special pointer types like [`PinMut`],
 /// which "pin" the type in place by not allowing it to be moved out of them.
+/// See the [`pin module`] documentation for more information on pinning.
 ///
 /// Implementing this trait lifts the restrictions of pinning off a type,
 /// which then allows it to move out with functions such as [`replace`].
@@ -632,6 +633,7 @@ unsafe impl<'a, T: ?Sized> Freeze for &'a mut T {}
 ///
 /// [`replace`]: ../../core/mem/fn.replace.html
 /// [`PinMut`]: ../pin/struct.PinMut.html
+/// [`pin module`]: ../../alloc/pin/index.html
 #[unstable(feature = "pin", issue = "49150")]
 pub auto trait Unpin {}