add link to std pin docs

Signed-off-by: Yoshua Wuyts <yoshuawuyts@gmail.com>

Author: yoshuawuyts

src/pin.rs

@@ -1,4 +1,6 @@
 //! Types that pin data to its location in memory.
+//!
+//! For more documentation see [`std::pin`](https://doc.rust-lang.org/std/pin/index.html).
 
 #[doc(inline)]
 pub use std::pin::Pin;