ping: Document this module.

src/ping.rs

@@ -5,9 +5,15 @@
 // 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 iq::IqGetPayload;
 
-generate_empty_element!(Ping, "ping", PING);
+generate_empty_element!(
+    /// Represents a ping to the recipient, which must be answered with an
+    /// empty `<iq/>` or with an error.
+    Ping, "ping", PING
+);
 
 impl IqGetPayload for Ping {}