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,