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

src/timer.rs

@@ -76,6 +76,9 @@
 //! [runtime]: ../runtime/struct.Runtime.html
 //! [tokio-timer]: https://docs.rs/tokio-timer
 //! [ext]: ../util/trait.FutureExt.html#method.deadline
+//! [Deadline]: struct.Deadline.html
+//! [Delay]: struct.Delay.html
+//! [Interval]: struct.Interval.html
 
 pub use tokio_timer::{
     Deadline,