Update model.js

Add `extend()` method to API docs.

lib/model.js

@@ -99,8 +99,10 @@ module.exports = function(registry) {
 
   Model.registry = registry;
 
-  /*!
+  /**
-   * Called when a model is extended.
+   * The `loopback.Model.extend()` method calls this when you create a model that extends another model.
+   * Add any setup or configuration code you want executed when the model is created.  
+   * See  [Setting up a custom model](http://docs.strongloop.com/display/LB/Extending+built-in+models#Extendingbuilt-inmodels-Settingupacustommodel).
    */
 
   Model.setup = function() {