message_correct: Document this module.

src/message_correct.rs

@@ -4,10 +4,17 @@
 // 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_element!(Replace, "replace", MESSAGE_CORRECT,
-attributes: [
+#![deny(missing_docs)]
+
+generate_element!(
+    /// Defines that the message containing this payload should replace a
+    /// previous message, identified by the id.
+    Replace, "replace", MESSAGE_CORRECT,
+    attributes: [
+        /// The 'id' attribute of the message getting corrected.
         id: String = "id" => required,
-]);
+    ]
+);
 
 #[cfg(test)]
 mod tests {