chatstates: Prevent compilation if the module isn’t properly documented.

src/chatstates.rs

@@ -4,6 +4,8 @@
 // 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 try_from::TryFrom;
 
 use minidom::Element;