let os::Stack's docs shine through the facade

src/os/mod.rs

@@ -1,9 +1,5 @@
 // Copyright (c) 2015, edef <edef@edef.eu>
 // See the LICENSE file included in this distribution.
-//! This module provides stacks using the operating system's
-//! anonymous memory mapping facility, usually `mmap`.
-//! The stacks it provides come with a guard page, which is not included
-//! in the stack limit.
 
 extern crate std;
 use core::prelude::*;
@@ -11,6 +7,10 @@ use self::std::io::Error as IoError;
 use stack;
 mod sys;
 
+/// This object represents a stack from the operating system's
+/// anonymous memory mapping facility, usually `mmap`.
+/// The stack it provides comes with a guard page, which is not included
+/// in the stack limit.
 #[allow(raw_pointer_derive)]
 #[derive(Debug)]
 pub struct Stack {