util: add links to tokio-util + example to BytesCodec (#2207)

tokio-util/src/codec/encoder.rs

@@ -2,21 +2,27 @@ use bytes::BytesMut;
 use std::io;
 
 /// Trait of helper objects to write out messages as bytes, for use with
-/// `FramedWrite`.
+/// [`FramedWrite`].
+///
+/// [`FramedWrite`]: crate::codec::FramedWrite
 pub trait Encoder {
     /// The type of items consumed by the `Encoder`
     type Item;
 
     /// The type of encoding errors.
     ///
-    /// `FramedWrite` requires `Encoder`s errors to implement `From<io::Error>`
+    /// [`FramedWrite`] requires `Encoder`s errors to implement `From<io::Error>`
     /// in the interest letting it return `Error`s directly.
+    ///
+    /// [`FramedWrite`]: crate::codec::FramedWrite
     type Error: From<io::Error>;
 
     /// Encodes a frame into the buffer provided.
     ///
     /// This method will encode `item` into the byte buffer provided by `dst`.
-    /// The `dst` provided is an internal buffer of the `Framed` instance and
+    /// The `dst` provided is an internal buffer of the [`FramedWrite`] instance and
     /// will be written out when possible.
+    ///
+    /// [`FramedWrite`]: crate::codec::FramedWrite
     fn encode(&mut self, item: Self::Item, dst: &mut BytesMut) -> Result<(), Self::Error>;
 }