change an XXX to a FIXME

www/common/outer/cache-store.js

@@ -97,7 +97,7 @@ define([
     var checkCheckpoints = function (array) {
         if (!Array.isArray(array)) { return; }
         // Keep the last 100 messages
-        if (array.length > 100) { // XXX 4.11.0
+        if (array.length > 100) { // FIXME this behaviour is only valid for chainpad-style documents
             array.splice(0, array.length - 100);
         }
         // Remove every message before the first checkpoint