Add some short docstrings for types. (#225)

These types were missing a short docstring, leaving the comment space to
their right blank on the crate page.

Also, this takes advantage of intra-rustdoc links to link terms:
https://github.com/rust-lang/rfcs/blob/master/text/1946-intra-rustdoc-links.md

src/resolve.rs

@@ -3,6 +3,7 @@ use std::io::Result as IoResult;
 use std::net::{SocketAddr, ToSocketAddrs};
 use std::sync::Arc;
 
+/// A custom resolver to override the default DNS behavior.
 pub trait Resolver: Send + Sync {
     fn resolve(&self, netloc: &str) -> IoResult<Vec<SocketAddr>>;
 }