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;