Add comment with explanation for independed pass

Author: L0stSoul

lib/csscomb.js

@@ -71,6 +71,10 @@ Comb.prototype = {
      * @returns {Array}
      */
     processTree: function(tree) {
+
+        // We walk across complete tree for each handler,
+        // because we need strictly maintain order in which handlers work,
+        // despite fact that handlers work on different level of the tree.
         this._handlers.forEach(function(handler) {
             this.processNode(['tree', tree], 0, handler);
         }, this);