General rustdoc improvements (#450)

* Normalize links to docs.rs/CRATE/M.N/...

docs.rs is smart enough to show docs for the latest M.N.P release when
M.N is used in the link. For example:

  https://docs.rs/mio/0.6/mio/struct.Poll.html

..will show mio 0.6.14 and later docs. While using the `M.N.*`
(ASTERISK) syntax also works, `M.N` is the more common usage, so
standarize a few existing links to that format.

* Fix missing or malformed rustdoc links

* executor lib rustdoc minor format change

* Promote tokio-threadpool crate level comments to rustdoc

* Replace hidden tokio::executor::thread_pool docs with deprecation note

* Fix typo/simplify util module rustdoc

* Reuse some tokio::executor::thread_pool rustdoc for the crate

Relates to #421

tokio-timer/src/timer/handle.rs

@@ -33,6 +33,7 @@ thread_local!(static CURRENT_TIMER: RefCell<Option<Handle>> = RefCell::new(None)
 /// This function panics if there already is a default timer set.
 ///
 /// [`Delay`]: ../struct.Delay.html
+/// [`Delay::new`]: ../struct.Delay.html#method.new
 pub fn with_default<F, R>(handle: &Handle, enter: &mut Enter, f: F) -> R
 where F: FnOnce(&mut Enter) -> R
 {

tokio-timer/src/timer/mod.rs

@@ -26,6 +26,7 @@
 //! [`Delay`]: ../struct.Delay.html
 //! [`Now`]: trait.Now.html
 //! [`Now::now`]: trait.Now.html#method.now
+//! [`SystemNow`]: struct.SystemNow.html
 
 // This allows the usage of the old `Now` trait.
 #![allow(deprecated)]