Fix phpdoc for constants in db\Schema [skip ci] (#12418)

framework/db/Schema.php

@@ -37,9 +37,7 @@ use yii\caching\TagDependency;
  */
 abstract class Schema extends Object
 {
-    /**
-     * The following are the supported abstract column data types.
-     */
+    // The following are the supported abstract column data types.
     const TYPE_PK = 'pk';
     const TYPE_UPK = 'upk';
     const TYPE_BIGPK = 'bigpk';