Add doc comment

Author: Steven Orvell

lib/legacy/class.js

@@ -303,9 +303,17 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       }
     }
 
-    __attributeReaction(name, oldValue, value) {
+    /**
+     * Processes an attribute reaction when the `legacyNoObservedAttributes`
+     * setting is in use.
+     * @param {string} name Name of attribute that changed
+     * @param {?string} old Old attribute value
+     * @param {?string} value New attribute value
+     * @return {void}
+     */
+    __attributeReaction(name, old, value) {
       if ((this.__dataAttributes && this.__dataAttributes[name]) || name === DISABLED_ATTR) {
-        this.attributeChangedCallback(name, oldValue, value);
+        this.attributeChangedCallback(name, old, value);
       }
     }