narrowsReducer [nfc]: Add a few comments.

These comments have been true for a while; I just noticed that they
were missing while working on the adjacent code.

src/chat/narrowsReducer.js

@@ -66,7 +66,15 @@ const eventNewMessage = (state, action) => {
         return; // i.e., continue
       }
 
+      // Don't add a message to the end of the list unless we know
+      // it's the most recent message, i.e., unless we know we're
+      // currently looking at (caught up with) the newest messages in
+      // the narrow.
       const isCaughtUp = action.caughtUp[key] && action.caughtUp[key].newer;
+      // Don't add a message that's already been added. It's probably
+      // very rare for a message to have already been added when we
+      // get an EVENT_NEW_MESSAGE, and perhaps impossible. (TODO:
+      // investigate?)
       const messageDoesNotExist = value.find(id => action.message.id === id) === undefined;
       if (isCaughtUp && messageDoesNotExist) {
         stateMutable.set(key, [...value, message.id]);