More commenting on the creation of guest filters

lib/sync.js

@@ -649,6 +649,8 @@ SyncApi.prototype._getGuestFilter = function() {
     if (!guestRooms) {
         return "{}";
     }
+    // we just need to specify the filter inline if we're a guest because guests
+    // can't create filters.
     return JSON.stringify({
         room: {
             timeline: {