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/agent.rs

@@ -12,6 +12,7 @@ use {
     cookie_store::CookieStore,
 };
 
+/// Accumulates options towards building an [Agent].
 #[derive(Debug)]
 pub struct AgentBuilder {
     config: AgentConfig,

src/lib.rs

@@ -231,7 +231,7 @@ pub use serde_json::{to_value as serde_to_value, Map as SerdeMap, Value as Serde
 use once_cell::sync::Lazy;
 use std::sync::atomic::{AtomicBool, Ordering};
 
-/// Creates an agent builder.
+/// Creates an [AgentBuilder].
 pub fn builder() -> AgentBuilder {
     AgentBuilder::new()
 }
@@ -253,7 +253,7 @@ pub fn is_test(is: bool) -> bool {
     return x;
 }
 
-/// Agents are used to keep state between requests.
+/// Agents are used to hold configuration and keep state between requests.
 pub fn agent() -> Agent {
     #[cfg(not(test))]
     if is_test(false) {

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>>;
 }