docs: update API documentation for some crates (#1380)

Updates API documentation for

- tokio-buf
- tokio-codec
- tokio-current-thread
- tokio-executor

tokio-codec/src/encoder.rs

@@ -3,9 +3,6 @@ use std::io;
 
 /// Trait of helper objects to write out messages as bytes, for use with
 /// `FramedWrite`.
-
-// Note: We can't deprecate this trait, because the deprecation carries through to tokio-codec, and
-// there doesn't seem to be a way to un-deprecate the re-export.
 pub trait Encoder {
     /// The type of items consumed by the `Encoder`
     type Item;

tokio-codec/src/lib.rs

@@ -7,13 +7,12 @@
 //!
 //! Contains adapters to go from streams of bytes, [`AsyncRead`] and
 //! [`AsyncWrite`], to framed streams implementing [`Sink`] and [`Stream`].
-//! Framed streams are also known as [transports].
+//! Framed streams are also known as transports.
 //!
-//! [`AsyncRead`]: #
-//! [`AsyncWrite`]: #
-//! [`Sink`]: #
-//! [`Stream`]: #
-//! [transports]: #
+//! [`AsyncRead`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncRead.html
+//! [`AsyncWrite`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncWrite.html
+//! [`Sink`]: https://docs.rs/futures-sink-preview/*/futures_sink/trait.Sink.html
+//! [`Stream`]: https://docs.rs/futures-core-preview/*/futures_core/stream/trait.Stream.html
 
 #[macro_use]
 mod macros;