Add api modules doc comments

Author: strosek

src/api/email.rs

@@ -1,3 +1,5 @@
+//! Module with client and endpoint functions for the Email channel.
+
 use std::collections::HashMap;
 use std::io;
 

src/api/sms.rs

@@ -1,3 +1,5 @@
+//! Module with client and endpoint functions for the SMS channel.
+
 use std::collections::HashMap;
 
 use validator::Validate;

src/api/whatsapp.rs

@@ -1,3 +1,5 @@
+//! Module with client and endpoint functions for the WhatsApp channel.
+
 use std::collections::HashMap;
 
 use reqwest::{Method, Response};

src/model/email.rs

@@ -1,3 +1,5 @@
+//! Models for calling Email endpoints.
+
 use serde_derive::{Deserialize, Serialize};
 use validator::Validate;