idle: Document this module.

src/idle.rs

@@ -4,12 +4,18 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+#![deny(missing_docs)]
+
 use date::DateTime;
 
-generate_element!(Idle, "idle", IDLE,
+generate_element!(
+    /// Represents the last time the user interacted with their system.
+    Idle, "idle", IDLE,
     attributes: [
+        /// The time at which the user stopped interacting.
         since: DateTime = "since" => required,
-]);
+    ]
+);
 
 #[cfg(test)]
 mod tests {