attention: Document this module.

src/attention.rs

@@ -4,7 +4,12 @@
 // 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/.
 
-generate_empty_element!(Attention, "attention", ATTENTION);
+#![deny(missing_docs)]
+
+generate_empty_element!(
+    /// Requests the attention of the recipient.
+    Attention, "attention", ATTENTION
+);
 
 #[cfg(test)]
 mod tests {