Added phpdoc for ContentDecorator

framework/widgets/ContentDecorator.php

@@ -11,6 +11,32 @@ use yii\base\InvalidConfigException;
 use yii\base\Widget;
 
 /**
+ * ContentDecorator records all output between [[begin()]] and [[end()]] calls, passes it to the given view file
+ * as `$content` and then echoes rendering result.
+ *
+ * ```php
+ * <?php ContentDecorator::begin([
+ *     'viewFile' => '@app/views/layouts/base.php',
+ *     'params' => [],
+ *     'view' => $this,
+ * ]) ?>
+ *
+ * some content here
+ *
+ * <?php ContentDecorator::end() ?>
+ * ```
+ *
+ * There are [[\yii\base\View::beginContent()]] and [[\yii\base\View::endContent()]] wrapper methods in the
+ * [[\yii\base\View]] component to make syntax more friendly. In the view these could be used as follows:
+ *
+ * ```php
+ * <?php $this->beginContent('@app/views/layouts/base.php') ?>
+ *
+ * some content here
+ *
+ * <?php $this->endContent() ?>
+ * ```
+ *
  * @author Qiang Xue <qiang.xue@gmail.com>
  * @since 2.0
  */