sync: polish and update API doc examples (#1398)

- Remove `poll_*` fns from some of the sync types.
- Move `AtomicWaker` and `Lock` to the root of the `sync` crate.

tokio-timer/src/timer/entry.rs

@@ -10,7 +10,7 @@ use std::sync::{Arc, Weak};
 use std::task::{self, Poll};
 use std::time::{Duration, Instant};
 use std::u64;
-use tokio_sync::task::AtomicWaker;
+use tokio_sync::AtomicWaker;
 
 /// Internal state shared between a `Delay` instance and the timer.
 ///