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-io/src/codec/mod.rs

@@ -104,7 +104,7 @@ pub mod length_delimited {
     //! [`FramedRead`] adapts an [`AsyncRead`] into a `Stream` of [`BytesMut`],
     //! such that each yielded [`BytesMut`] value contains the contents of an
     //! entire frame. There are many configuration parameters enabling
-    //! [`FrameRead`] to handle a wide range of protocols. Here are some
+    //! [`FramedRead`] to handle a wide range of protocols. Here are some
     //! examples that will cover the various options at a high level.
     //!
     //! ## Example 1
@@ -370,7 +370,7 @@ pub mod length_delimited {
     //! [`AsyncRead`]: ../../trait.AsyncRead.html
     //! [`AsyncWrite`]: ../../trait.AsyncWrite.html
     //! [`Encoder`]: ../trait.Encoder.html
-    //! [`BytesMut`]: https://docs.rs/bytes/~0.4/bytes/struct.BytesMut.html
+    //! [`BytesMut`]: https://docs.rs/bytes/0.4/bytes/struct.BytesMut.html
 
     pub use ::length_delimited::*;
 }