adjusted guide docs for SchemaBuilder

docs/guide/db-migrations.md

@@ -94,11 +94,12 @@ when you upgrade the database with this migration, while the `down()` method is
 The following code shows how you may implement the migration class to create a `news` table: 
 
 ```php
+<?php
 
 use yii\db\Schema;
 use yii\db\Migration;
 
-class m150101_185401_create_news_table extends \yii\db\Migration
+class m150101_185401_create_news_table extends Migration
 {
     public function up()
     {
@@ -140,22 +141,23 @@ to `Schema::TYPE_STRING` to specify that the column cannot be null.
 > Info: The mapping between abstract types and physical types is specified by 
   the [[yii\db\QueryBuilder::$typeMap|$typeMap]] property in each concrete `QueryBuilder` class.
   
-Since 2.0.5 schema builder which provides more convenient way defining column schema was introduced so migration above
+Since version 2.0.5 schema builder which provides more convenient way defining column schema was introduced so migration above
 could be written like the following:
 
 ```php
+<?php
 
 use yii\db\Schema;
 use yii\db\Migration;
 
-class m150101_185401_create_news_table extends \yii\db\Migration
+class m150101_185401_create_news_table extends Migration
 {
     public function up()
     {
         $this->createTable('news', [
-            'id' => Schema::primaryKey(),
-            'title' => Schema::string()->notNull(),
-            'content' => Schema::text(),
+            'id' => $this->primaryKey(),
+            'title' => $this->string()->notNull(),
+            'content' => $this->text(),
         ]);
     }
 
@@ -167,6 +169,8 @@ class m150101_185401_create_news_table extends \yii\db\Migration
 }
 ```
 
+A list of all available methods for defining the column types is available in the API documentation of [[yii\db\SchemaBuilderTrait]].
+
 
 ### Transactional Migrations <span id="transactional-migrations"></span>