doc improvement [skip ci]

docs/guide/concept-di-container.md

@@ -76,17 +76,44 @@ $container->get('Foo', [], [
 ### PHP Callable Injection <span id="php-callable-injection"></span>
 
 In this case, the container will use a registered PHP callable to build new instances of a class.
+Each time when [[yii\di\Container::get()]] is called, the corresponding callable will be invoked.
 The callable is responsible to resolve the dependencies and inject them appropriately to the newly
 created objects. For example,
 
 ```php
 $container->set('Foo', function () {
-    return new Foo(new Bar);
+    $foo = new Foo(new Bar);
+    // ... other initializations ...
+    return $foo;
 });
 
 $foo = $container->get('Foo');
 ```
 
+To hide the complex logic for building a new object, you may use a static class method to return the PHP
+callable. For example,
+
+```php
+class FooBuilder
+{
+    public static function build()
+    {
+        return function () {
+            $foo = new Foo(new Bar);
+            // ... other initializations ...
+            return $foo;
+       };        
+    }
+}
+
+$container->set('Foo', FooBuilder::build());
+
+$foo = $container->get('Foo');
+```
+
+As you can see, the PHP callable is returned by the `FooBuilder::build()` method. By doing so, the person
+who wants to configure the `Foo` class no longer needs to be aware of how it is built.
+
 
 Registering Dependencies <span id="registering-dependencies"></span>
 ------------------------

docs/guide/concept-service-locator.md

@@ -61,8 +61,10 @@ If you call [[yii\di\ServiceLocator::get()]] with an invalid ID, an exception wi
 
 
 Because service locators are often being created with [configurations](concept-configurations.md),
-a writable property named [[yii\di\ServiceLocator::setComponents()|components]] is provided. This allows you to configure and register multiple components at once. The following code shows a configuration array
-that can be used to configure an application, while also registering the "db", "cache" and "search" components:
+a writable property named [[yii\di\ServiceLocator::setComponents()|components]] is provided. This allows you 
+to configure and register multiple components at once. The following code shows a configuration array
+that can be used to configure a service locator (e.g. an [application](structure-applications.md) with 
+the "db", "cache" and "search" components:
 
 ```php
 return [
@@ -76,36 +78,40 @@ return [
         ],
         'cache' => 'yii\caching\ApcCache',
         'search' => function () {
-            return new app\components\SolrService;
+            $solr = new app\components\SolrService('127.0.0.1');
+            // ... other initializations ...
+            return $solr;
         },
     ],
 ];
 ```
 
-In complex cases it's a good idea to create extra class to hide configuration complexity:
+In the above, there is an alternative way to configure the "search" component. Instead of directly writing a PHP
+callback which builds a `SolrService` instance, you can use a static class method to return such a callback, like
+shown as below:
 
 ```php
-class FacebookSDK {
-    public static function service($config) {
-        return function() use $config {
-             // build the object here
-             return $object;
+class SolrServiceBuilder
+{
+    public static function build($ip)
+    {
+        return function () use ($ip) {
+            $solr = new app\components\SolrService($ip);
+            // ... other initializations ...
+            return $solr;
         };
     }
 }
-```
-
-It could be used as follows:
 
-```php
 return [
     // ...
     'components' => [
         // ...
-        'facebook' => FacebookSDK::service([
-            'secret' => 'theverysecret',
-            // ...
-        ]),
+        'search' => SolrServiceBuilder::build('127.0.0.1'),
     ],
 ];
 ```
+
+This alternative approach is most preferable when you are releasing a Yii component which encapsulates some non-Yii
+3rd-party library. You use the static method like shown above to represent the complex logic of building the
+3rd-party object, and the user of your component only needs to call the static method to configure the component.