Make EmailFormat trait private (#535)

It does not need to be exposed.

src/address/mod.rs

@@ -1,3 +1,5 @@
+//! Email addresses
+
 #[cfg(feature = "serde")]
 mod serde;
 

src/error.rs

@@ -1,3 +1,5 @@
+//! Error type for email messages
+
 use std::{
     error::Error as StdError,
     fmt::{self, Display, Formatter},

src/message/mod.rs

@@ -265,7 +265,8 @@ use crate::{
 
 const DEFAULT_MESSAGE_ID_DOMAIN: &str = "localhost";
 
-pub trait EmailFormat {
+/// Something that can be formatted as an email message
+trait EmailFormat {
     // Use a writer?
     fn format(&self, out: &mut Vec<u8>);
 }

src/transport/smtp/transport.rs

@@ -13,6 +13,7 @@ use crate::Transport;
 
 #[allow(missing_debug_implementations)]
 #[derive(Clone)]
+/// Transport using the SMTP protocol
 pub struct SmtpTransport {
     #[cfg(feature = "r2d2")]
     inner: Pool<SmtpClient>,