Add a XXX caveat to #346 fix.

packages/mongo-livedata/collection.js

@@ -179,6 +179,8 @@ Meteor.Collection._rewriteSelector = function (selector) {
   var ret = {};
   _.each(selector, function (value, key) {
     if (value instanceof RegExp) {
+      // XXX should also do this translation at lower levels (eg if the outer
+      // level is $and/$or/$nor, or if there's an $elemMatch)
       ret[key] = {$regex: value.source};
       var regexOptions = '';
       // JS RegExp objects support 'i', 'm', and 'g'. Mongo regex $options